opened at any time when required, and so arranged that the jar containing the meat could be returned to the seller. It would also be an advantage to persons in better circumstances to have such a supply of animal food at command at all times and seasons. Beyond these advantages there was another-viz., that this mode of preservation would be most convenient to the preserver, who would be able to divide the parts sent over into equal portions, and export those parts only which were serviceable to the consumer.

After the bottles were charged with the food and with the preservative, the stoppers were firmly inserted, tied down with wire, and thoroughly sealed. The jars were then placed one upon another in these long zine tubes, cased in thick felt. Each tube would hold four bottles, separated from each other by a wad of felt. Then the zinc tubes were soldered down, and were packed side by side in a partitioned wooden case, as you see there, and, packed in this manner, they were ready either to be placed aside at home in a heated room or to be sent on a voyage. The common plan adopted was to prepare two cases alike, keep one at home, send the other abroad, and compare the results of the one with the other, when the return specimens were brought back for examination.

from which two half-inch metal tubes, furnished with closing screw stoppers, stand out at right angles. One of these tubes runs straight into the cylinder, the other, after entering the cylinder, turns and runs nearly to the bottom of the cylinder. The whole case dy be compared to a Wolfe's bottle on a large scale.

diffusion of the vapour of the fluid into the body, that turned on two chief conditions-the boiling point of the fluid itself, and the temperature of the surrounding air. The whole plan promised at first so well that my expectations as to its perfect success, I admit, were very great. How it answered, and what value may yet be attached to it, will all appear in proper time and place. I am merely giving details of processes at this moment.

In using this preserving case the same plan is followed as with the glass bottle, with this difference that, if a gas is to be used as the preserva- One more of these details is all I need trouble tive, the specimens are introduced, the lid is you with further. It relates to the mode of securely closed, and the gas is driven into the keeping observations. These records were in two closed case through the long descending tube, series; one was a series of minute and special the shorter tube being for the time left open. observations, designed for my own future guidance When the specimens have thus been left for a in research; the other was a general record of resufficient time to the action of the gas, the supply-sults, designed as a guide to immediate practical ing pipe is disconnected, and the two tubes of the applications. The last-named order of observacylinder are closed by the insertion of their closing tions were reduced to the greatest simplicity, so that screws. The lid of the case is finally sealed down any intelligent assistant could take down permaby running a layer of cement or solder into the nently without committing any serious mistake. shelving groove. As a rule, however, I supervised every examination, and dictated the facts as they were observed. To secure uniformity a printed form was used for recording the facts presented.

I cannot report that this addition of weight gave any real addition to the security from putrefactive change.

In using preservative gases I found at first not a little difficulty, after preparing the gas, in keeping it so as to measure it carefully out, in each experiment, without bringing it into contact with water, which, in the case of all soluble gases, would absorb some gas and so vitiate the result. I tried first to measure the quantity of gas by means of a small gas meter, but that was not satisfactory. At last I constructed the instrument which is before us, and which, though it is still not quite perfect, answers better than any other instrument I have been able to find or make. In constructing a second instrument of the kind I could render it quite perfect. This instrument is nothing more than a large inverted syringe. The cylinder is of glass, and is graduated into cubic inches and tens of inches. The cylinder is fixed on an iron tripod, and the piston works from below, the handle of the piston coming down between the legs of the tripod. The upper lid or cover of the cylinder has in it an opening, which receives a stopper, and it is also furnished with a side tap. When the cylinder is about to be used the piston is pushed upwards until it comes to the lower face of the cover of the cylinder at the upper part. Either the opening in the cover or the opening from the tap is then connected with the gas that is about to be used, the piston is slowly drawn down, and the quantity of gas required is drawn into the cylinder. This accomplished, the tpening in the cover of the cylinder is closed, and the transference of the gas to the specimen can be made at leisure. The transference is conducted by connecting the tap of the cylinder with a tube leading to the jar which contains the specimen that has to be preserved. After the connection is made, the tap is turned open, and by mere pressure of the piston upwards, the precise quantity of gas that may be desired is forced into the preserving jar. In constructing a new gas measuring apparatus of this kind, I should follow the same kind of principle, but I should not invert the cylinder, I should let it fill from the bottom. I should let the piston descend, as in the common syringe, and I should regulate its descent, and

thereby the pressure on the gas, by using a screw piston which would be more accurate and steady in its action.

So much for the modes by which the glass jars were charged; I have yet to touch on the plans that were tried for preserving quarters or whole carcases of dead animals intended for food. When

the carcase, say of a sheep, was divided into parts, it was placed in one of these light iron cases, which is made fo hold fifty-four pounds of meat in quarters or joints. It is a strong iron case with an arched lid, and the lid is made to drop into a shelving dropped into its position, and then, by a half groove below the rim of the cylinder. The lid is turn round, is made to slip into a bayonet joint, which holds it firmly when required. There is one other arrangement about this larger preserving vessel. It has in it two openings at the top

I

I have yet one other plan to explain, which is more novel than those plans which have been described, and I wish I could say was more perfect. It is one of those plans which promise well in theory, and which are, I believe, suggested to test and to correct the temper and endurance of men who follow experimental pursuits. The idea was that, taking advantage of diffusion of vapours of an antiseptic character, I could diffuse them through the arteries of an animal recently dead, and make them, like a preserving breath, penetrate into the minutest structures, and so preserve. I thought at the same time that, by using the store of the vapour in the condensed state of a liquid, I could so arrange the process of evaporation that, when the temperature was high, the diffusion would be intensified, and when the temperature was low the diffusion would be reduced. In this way, it seemed to me, a self-regulating, I had almost said automatic, plan of preservation would be secured.

In order to put to experiment the value of this idea, I constructed this flask of metal. The flask is capable of holding ten fluid ounces, and it is made after the plan of an exciseman's ink bottle. There is, that is to say, projecting into it, from a central neck, a conical tube. When a fluid, say chloroform as a ready example, is put into this flask, it will not run out as a fluid in whatever position the flask is held, but in all positions the vapour of chloroform will readily escape, and the rate of the escape of vapour will be governed largely by the external temperature. The flask then, so constructed, was charged with volatile preservative fluid, and was ready for use.

OBLIQUE ILLUMINATION AND
MEANS OF OBTAINING IT.*

By R. HITCHCOCK, M.D.

is generally understood that the appearance of

upon the character of the illumination as upon the objective. Therefore, any improved method of controlling the light demands attention, and I have chosen two recent accessories for consideration in this place; one devised by Dr. J. J. Woodward, which recommends itself from its simplicity and cheapness; the other by Dr. James Edmunds, described last year before the Royal Society, which, from the accounts we hear of it, is destined to become a valuable accessory.

Dr. Woodward's apparatus is figured below, having been reduced from the M. M. J.), and may be thus briefly described: Just beneath the slide, F, and connected with it by oil of cloves or other highly retractive medium, is a truncated right-angled glass prism, resting upon a similar brass prism supported on the sub-stage. D is a shutter, parallel to the side of the prism as shown, with a small hole, E, about like The side of the prism is covered a large pinhole. with black paper with a co responding pinhole. A balsam-mounted object, viewed with an immersion lens of sufficient angular aperture, can thus be seen with sunlight from the plane mirror at an angle of 45° from the axis. It is only by. some such plan as this that we are able to obtain light of this obliquity in examining balsam mounted objecte.

We now pass to the more costly appliance of Mr. Edmunds. This consists of a paraboloid lens, with the front cut off flat and polished. The extent to which the apex is cut off depends upon the conditions of use, and may be one-twelfth of an inch below its interual focus, or barely one-fiftieth of an inch

below

the

This lens is used beneath the slide, in fluid contact with it, and the thickness of the slide determines the place of cutting; the object being to secure a lens The next step consisted in applying the flask to which shall have its focus for parallel rays just at the dead carcase, so as to cause diffusion of the Diaphragms and shutters of any kind may be arranged upper surface of the slide when in fluid contact. vapour through the tissues. To effect this purpose, beneath the lens, and any rays can thus be selected the carcases of sheep, which had been killed in the for use at pleasure. Such a lens has peculiar proordinary way, were suspended by the hinder perties, which we should observe in order to underquarters, as if they were about to be flayed in the stand its action and advantages. Excluding the usual system of dressing. Instead of the dressing being done, the abdomen of the dead animal was more central rays, light entering from below will opened by the butcher, and the whole of the intes-internal reflection. This is not true of the Wenham not pass out again into air above, but suffer total tines were removed, a tight ligature being first paraboloid in common use, and this very fact makes applied, so as to include all the blood vessels which the new lens the more valuable for use with high had fed the intestines, and to close them. These vessels tied, an opening an inch long was made into powers for resolutions. the abdominal aorta, and a metal tube, having two branches standing nearly at right angles from the centre tube, was tied firmly into the aorta by its branches, one branch being directed towards the upper portion or trunk of the carcase, the other towards the hind quarters. The branches securely tied into the blood vessel, the central tube was closely connected with the neck of the flask containing the preservative fluid; the flask was fixed in the cavity of the abdomen; the opening that had been made into the cavity was neatly closed by stitches; the vessels of the neck of the animal were

tied, after the head had been removed, and the carcase was left in order that we might observe from it what changes would follow.

The whole process was so simple that an ordinary butcher was able to carry it out with the greatest ease, and in much less time than it took him to 66 dress another carcase for the market. You

will at once see what was done and what was attempted to be done. That which was done was flask was so placed in relation to the body of the this:-The vapour escaping from the neck of the dead animal that it could diffuse through the whole of the arterial system. The body of the animal stood in the light of a receiver to the retort which held the preservative fluid. As to the rate of

of glycerine. The light now passes up through Place a slide upon the stage, and interpose a drop glycerine and slide, and can be focussed on the upper

surface.

and with rays of great obliquity only. Obviou-ly Any object placed upon the slide is well lighted, this paraboloid gives us a perfect dark ground effect, objective can take them up, and with a dry one the the rays being so very oblique that no immersion only rays that pass to the objective must be radiated

from the object itself.

new

It certainly appears, then, that this superiority over any other yet devised, and this "immersion paraboloid" possesses certain points of seems to be borne out by the results of experience. The fact should not be overlooked, however, that Mr. Wenham claims to have made a similar instrument as early as the year 1856.

My purpose in describing these instruments is to and which has been too much ignored by microscoopen a question which seems pertinent at this time, above? Perhaps it is too early to settle the question pists as a rule: what kind of illumination is the best-that from below the stage or that from now, but it certainly is time to consider it carefully. A paper read before the New York Microscopical

Society

First, let us ask, are these, and the many other substage accessories, of sterling value to the observer? There are those who would discard everything beneath the stage but the mirror, and among these are men of no mean authority in such matters. As for them the reason lies in this, that they believe it possible to resolve any test without any other accessory. Well, we must admit that this is true. No test has yet been proposed that cannot be resolved in this way and by an expert workman, without great difficulty. Nevertheless, while admitting the fact, is it enough to lead one to discard all other devices? Knowing, as we do, the great difficulties that often arise in making out the exact meaning of the appearance of minute structure under the microscope, should we rot always be glad to accept every mears in our power to modify and improve our illumination? It does not follow that because a test is resolved that we bare the best results our lens is capable of giving. It certainly appears from the discussion before the Quekett Club, where this new condenser was exbibited, that the appearance of these very tests was somewhat novel. Another point made by Mr. Edmunds tends to confirm an opinion of long standing. He suggested that "if we could, by new means of illumination, increase the resolving power of low-angled lenses, a vast gain would be made," and it seems already that he has done this to a certain extent by this very paraboloid.

Admitting the value of these accessories as aids to investgation, we return to our original questionwhat is the best kind of illumination ? Taking a difficult frustule of a distom let us observe the effect of various methods with the mirror. With direct light we can see the outline, with oblique light the fine markings, and as less and less light passes through the object in a line with the optical axis, the more distinct become the markings. We might justly conclude from this, that the proper method of illumination would not be by transmitted light.

In practice we only secure the best resolutions of lines or series of dots, when the line is of the utmost possible obliquity. Now, the ordinary way of look. ing at objects in every day life is by reflected light. We are easily deceived as to a transparent object when seen by transmitted light. Naturally this is also the case when using the microscope, and I would speak in favour of reflected light for this purpose. In fact, I am strongly inclined to the opinion that the most appropriate and best light for microscopical work is what comes from above the stage. What we want is a positive image of the object, and this cannot be obtained by the ordinary methods of working. One of the greatest obstacles to this kind of illumination has heretofore been the difficulty of throwing sufficient light upon the object with high powers. A 1-10th can now be used very successfully in this way with simply the mirror above the stage.

Practically, a positive image is also obtained, when the object is transparent, by means of very oblique light from such an instrument as we have described. It will be a difficult matter to show any difference, in effect, between a transparent body thus made self-radiant, and an opaque one illuminated from

above.

Moreover, it may be that the effect of very oblique light, giving a dark field, is really something more than we have suggested. Long ago, Mr. Wenham devised various means of obtaining oblique light for balsam or fluid mounts, and he considered that in many cases the light suffered total reflection from the surface of the cover-glass and was thrown down upon the object, thus in fact giving the effect of illumination from above: only, however, with dry objectives. It is often difficult, if not impossible, to say whether an image we see is truly positive or not. It seems not unlikely that sometimes the object seen by such oblique light really shows by the reflected light from the cover. These are old ideas, but they should not be forgotten entirely. There is one remark I wish to add here for the mere purpose of putting myself on record in this matter. I wish to

THE NEW FLYING MACHINE.

WE are now enabled to place before our readers the principal details of the new flying machine, which we described in general terms on p. 465 of Vol. XXVII. It will be remembered that two very successful ascents were made with the machine, and sufficient

FIG.I.

HI

H

FIG. 2

W

To the main driving wheel is secured a crank,

by means of which the machinery is operated. This bevelled wheel meshes with a bevelled pinion, which is secured to the top of the vertical shaft, Fig. 1. To the lower end of this shaft is secured a bevelled wheel, which is always in gear with one of the two bevelled wheels on the horizontal shaft, H. These two wheels are placed loosely on shaft, H, and gear at their lower edges with a bevelled wheel on the top of the shaft, to which the lifting propeller wheel is secured. Feathered upon the shaft, H, on the outer side of each wheel, is a clutch, which may be thrown in gear alternately with either wheel by means of a hand lever, placed within easy reach of the operator. By throwing one clutch in gear the propeller will be made to revolve in one direction, and lift the machine upward, and by throwing in the other clutch the shaft, H, will revolve in the opposite direction, and cause the machine to move backward.

The shaft, H, may be made in one continuous piece, or may be jointed, as here shown, and have its forward end project through the front of the frame to receive the bevelled wheel, S, Fig. 2. This wheel meshes with a similar wheel placed on the vertical shaft, V, journalled in suitable bearings, which wheel communicates motion to the wheel secured to the inner end of the shaft to which the guiding and propelling fan is secured. The frame carrying the propelling fan and the gear is pivoted, P, to the supports extending from the balloon; hence the fan can be moved around to either side. A treadle is placed near the foot of the operator, to which is connected a rod, the front of which has a rack formed upon it for the purpose of engaging with the wheel, W, secured to the top of the vertical shaft, V, to which the bearings of the propelling fan are secured. The operator, by pressing with his foot upon the treadle, can turn the propeiling fan to the front, or around to either side, or to any intermediate point between. The operator having started the main driving wheel, throws one of the clutches in gear, and the propeller (Fig. 1). causes the machine to rise upward, at the same time as the propelling fan causes the machine to move forward. By means of the treadle the operator causes the propelling fan to turn in any direction, and thereby makes the machine move straight ahead to either side, back, or turn completely round, as upon a pivot. By reversing the horizontal fan the machine will descend at any desired rate of speed, and the was done to show that properly worked out the front propeller (Fig. 2) may be made to stand idea was the most practical of any then in- still if desired. Although the latter is shown as vented. The patent was secured a few months having only a horizontal shaft, the patentees ago in this country by F. A. Lehmann, of may in some instances use a universal joint, and Washington, and C. Ritchel, of Corry, Pennsyl- thus employ it for assisting the machine to vania. The features claimed by the patentees rise vertically upward and to descend, as well as are 1st. Pivoting or journaling a propeller to move the machine backwards or forwards or wheel upon the front end of the machine, around in a circle. whereby the machine can be made to move either backward or forward, or turn to the right or left, thus enabling the operator to move the machine in any direction desired. 2nd. The combination of a balloon, an operating mechanism, a propeller wheel to raise the machine, and a propeller wheel that is pivoted upon the front end of the machine, so that it can be turned in any direction, and which moves the machine in any desired direction. Fig. 1 is a side elevation of the devices for operating the lifting wheel; and Fig. 2 is a detail view of the device for operating the guiding and propelling wheel upon the front end of the

machine.

The latter consists of a balloon of any shape, size, or construction, provided it has a lifting capacity sufficient to almost lift the apparatns together with at least one person, thus leaving very little for the lifting wheel to do. By thus giving the balloon such a lifting power all the more force may be applied to driving the machine back and forth and from side to side, and the descent will be more easy and gradual. Secured to the under side of this balloon, by means of suitable light strong rods and braces is a frame of any desired construction or material, for the support of the operator and the driving machinery. Resting upon two of the cross bars of this frame is the seat upon which the operator sits, and this seat is so located in respect to the machinery as to enable him to apply his whole strength to the propulsion and

management of the machine.

A NEW RHEOSTAT.*

LAST May for the Faradic as well as the con

I invented a new rheostat, which can

tinuous current. It recommends itself for simplicity. variable length is the only means for the resistance. durability, and cheapness. A line of plumbago of For the construction of the instrument the following few articles are required:-A small quantity of plumbago, a piece of paper, two binding screws terminating in a wire of lin. length, and 3-16in. thickness, two boards 6in. by 24in. one of them in.. the other in. thick, two common screws, shellac varnish.

FICE

and

To make the rheostat :-Lay out the thicker board, as shown in Fig. 1. The upper line should be 4in. long, the others proportionally; this should be divided into six equal parts, and the remaining four

By Dr. F. G. OEHME, in the Scientific American.

be worked (beauty, as usual, the stamp and seal of perfection) have made it the pet of the household. Now to r tarn to our cell, we see that its peculiar power depends on the fact that, however it be deformed, the product of the varying lengths A D, A E, Fig. 4, always remains constant. If when these points coincide, the distances AE and AD be taken equal to one foot and then the cell be moved again, when A D takes respectively the lengths 1, 3, 1. 1. &c., then A E will be found to assume the lengths 1, 1, 2, 3, &c., showing that the length of one is so governed by the length of the other that their product must remain constant.

Now Mr. Hart found that if he took four bars and made a linkage in which the adjacent sides are unequal and two cross as in the figure, and then took four points on the four links dividing the distances between the pivots in the same proportion, those points will always remain in a straight line and possess the peculiar property just adverted to, so that the product AD. AE is constant. So also is OE.OD, and also AD.DO and AE.EO. So we see immediately that we may employ Hart's cell of only four bars exactly as we employed Peaucellier's of six bars, and by fixing one of the points, as A, and pivoting our extra link to another, as D, we can get straight line motion with only five bars, which is the least number possible, as has been absolutely demonstrated.

The Quadruplane.

A beautiful and important extension of this discovery was made at the same time by Prof. Sylvester and Mr. Kempe. Prof. Sylvester has given anite an elaborate de-cription of it, but I use Mr. Kempe's own words as being simpler. "If we take the contra-parallelogram of Mr. Hart and bend the links at the four points which lie on the same straight line, through the same angle, the four points, instead of lying in the same straight line, will lie at the four argular points of a parall logram of constant angles-two the angle that the bars are bent through and the other two its supplement-and of constant area, so that the product of two adjacent

sides is constant."

If we keep the lettering of the last figure, take the holes or points in the middle of the links and bend them through a right angle as the simplest, we have the figure here given. The four holes now lie at the four corners of a right-angled parallelogram, Fig. 5, and the product of any two adjacent sides, as AD AE, is constant.

It follows that if A be fixed and D pivoted to the extremity of the extra link, whose other extremity is always pivoted to a point equidistant from A and D, the point, E, will describe a straight line differirg in direction from the line it described before the beading by precisely the same angle the bars have been bent through, in this chosen case by a right angle.

By looking at the figure it is seen that the apparatus, which for simplicity has been described as formed of four straight links which are afterwards bent, is really formed of four plane pieces on which appropriate points are chosen. This is why it is called the "Quadruplane" by Prof. Sylvester, who says: "The quadrupline gives the most general and available solution of the problem of exact parallel motion that has been discovered, or that can exist. I say the most available, for it is evident, in general, that piece work must possess the advantage of greater firmness and steadiness, from the more equal distribution of its strains, over ordinary linkwork."

The Plagiograph.

From the ordinary pantagraph familiar to mechanicians, an application of this same ideanamely, turning two of its links into pieces or planes gives a beautiful extension of it, called by Prof. Sylvester, its inventor, the plagiograph. "Like the pantagraph, it will enlarge or reduce fgures; but it will do more, it will turn them trough any required angle "Thus the plagiograph erables us to apply the principle of angular repetition (as, for in-tance, in making an ellipse with dimensions either fixed or varying it will, succes sively turn its axis to all points of the compass), to produce designs of complicated and captivating symmetry from any simple pattern or natural form, such as a flower or sprig. This should be found to Face a new and powerful implement in the hand of the pattern-des grer and architectural decorator.

they were able to operate with a radius of not more than 6ft. or 7ft. in length.

These are but the simplest of the innumerable applications contained in, and immediately suggested by, the new science of linkage. Only let the practical mechanician begin to make for himself models of those here described, and we guarantee him a rich harvest of unlooked-for results. In the words of its founder, "I feel a strong persuasion that when the inertia of our operative classes shall have been overcome, this application will prove to be but the signal, the first stroke of the tocsin, of an entire revolution to be wrought in every branch of construction."

IMPROVEMENTS IN STUDYING THE CHARACTERS OF MINERALS.*

BY H. C. SORBY, F.R.S.

N the following short paper I propose to describe method of studying the optical characters of the minerals which I treated at greater length in my address last year at the annual meeting at Plymoutht.

It is a curious example of how a method may be invented, and then lost sight of, that the determina tion of the index of refraction in the way there described was proposed by a French savant upwards of a hundred years ago. I have not yet consulted the original publication, but I very strongly suspect that the proposal was more theoretical than practical, and that with the instruments then at disposal the results were found to be so inexact that the whole system became obsolete and practically forgotten. I may, however, claim to have so modified the method, and brought the instrumental means to such perfection, as to make it fully equal to the requirements of practical mineralogy. Whilst speaking on this point it may be well to give an illustration of the accuracy with which it is possible to measure the index with the apparatus which I have now at disposal. Thus, in the case of a specimen of quartz, about 372in. thick, five different determinations of the index of the ordinary ray for the light transmitted by red glass, which corresponds to the solar line c, were 15513, 15531, 15524, 15531 and 1.5513, so that no observation differed more than an unit in the third place of decimals from the mean value, which may, therefore, be looked upon as true to the third place of decimals, assuming that the equation u needs no correction.

=

T

T d There was no difficulty in thus proving that there is a slight but well-marked difference in the index for different specimens. The mean for five was 1.5543, whereas, according to Rudberg, it is 1.5418. In a similar manner I found that my method minerals. After many very careful measurements invariably gave too high a result in the case of other I came to the conclusion that this can be satisfactorliy attributed to the spherical aberration due to the introduction of a transparent plate in front of the object-glass, as suggested by Professor Stokes. The amount of this error depends partly on the index of refraction, and partly on the special correction of each particular object-glass, and when great accuracy is desired it is necessary to construct a small table showing the amount that must be deducted in each case. I thus find that, when using my object-glass, if the index is about 1.5 I must deduct 0100, and, when 2'0, must deduct 0180.

Having thus shown how accurately the index may be measured, it may be well to briefly allude to some improvements in the apparatus. I find two cross lines in the focus of the eye lens very useful in keeping constant the focal adjustment of the eye itself. In adjusting the focus of any object it is always arranged so that the cross lines are also in sharp focus. Without this precaution there may be an important difference, according as the focus is adjusted by moving the object-glass up or down. I have also found it desirable to take the means of two or more sets of measurements made in slightly different parts of the scale, so as to eliminate any error due to imperfect graduation. This is easily managed by moving the fine adjustment. It is by adopting these precautions that I have been able to make such concordant and accurate measurements as those given above in the case of quartz, and to prove that the limit of error may be made very small.

Final y, we have seen that in using a linkage to When first I commenced to apply my method to draw a straight line, the di tance between the fixed the study of various minerals, with the view of compivots must always be the same as the length paring mathematical theory with observation, I soon of the extra Lak. Now if this distance is found that there were a few discrepancies. For Bot the same, the pencil-point describes, not some time I thought it just possible that these stra ght Laes, but circles. If the difference be might be due to errors in the measurements, but I Eight, the ans described will be of enormous found that these discrepancies became the more and magnitude, demeing in size as the difference more marked as by degrees I was able to remove increases. This propert, is o' very high importance every apparent source of error. The principal disin the merkan ra, arts for describing circles of large crepancy is in the case of bi-axial crystals like INCIDE. Prd By veter etes as examples some aragonite, but some are also met with in the case of erstar et je outside St. Paul's Cathedral, which uniaxial crystals. I have not yet been able to requiring repair, Mr. Penrose employed a Peaucellier thoroughly ascertain the laws which govern these od to cut out tempets in zine for the purpose. The l ru the steps is about it.; but to the great! evafort and delectation of his clerk of the works,

From the Mineralogical Magazine.

† See p. 6, Vol. XXVI.

special peculiarities, and no kind of explanation has yet suggested itself either to Professor Stokes or myself; and therefore it appears to me undesirable to enter more fully into the question, which relates more to the mathematical theory of light than to practical mineralogy. It may, however, be well to say that the discrepancy to which I refer is in the ratios of the values of the real and apparent indices.

My attention has been so much devoted to these interesting and important matters of detail, that I have had but little opportunity to further apply the method to the identification of doubtful minerals. It may, however, be well to give one illustration.

I had in my collection two six-sided prismatic crystals with oblique terminations. They had the general aspect of calcite, but then the six angles were obviously unequal, and polarised light at once showed that the axes of elasticity were very far from parallel and perpendicular to the axis of the prism. On examining them by my method I at once saw by the character of the images that I had before me an uniaxial crystal with powerful double refractions, and that the direction in which I could observe the indices was such as would give their true values, since there was no material lateral displacement of the images. I give below the three apparent indices, two real and one only apparent, and compare them with calcite :Ordinary Ray. Extraordinary Ray.

last week without accident. The following statistics THE Giffard great captive balloon was disinflated in connection with the enterprise may be interesting. The Tuileries grounds were opened to the public during 100 days, but the balloon was unable to work owing to the state of the atmosphere during 30 days. The number of ascents was 1,023, the number of passengers 34,000. The number of pioneer balloons sent up 25. of 840,000 francs was collected. During this period the sum The expenses of building the balloons, of machinery, and working, was a financial success.-Nature. reached about 500,000 francs, so that the enterprise

analogous material is torn into shreds and placed Manufacture of Celluloid.-Paper or other in a vat containing mixed sulphuric and nitric acid, the whole being thoroughly stirred. After a suit able time the contents of the vat are transferred to a centrifugal machine, in which the acid is thrown off. The now converted paper is then washed to completely remove the acid, and the pyrolina is banked under water until required. The plastic planed into sheets, which are converted into slabs composition is then consolidated and afterwards they are forced into apertures in the plate sustaining by heat and pressure, rendering them plastic, when the slab, and as it hardens it shrinks and produces slabs hold to the plate with great power. Celluloid a tension upon the intermediate plate, whereby the and other plastic compositions are similarly employed to coat and form various articles, such as combs; in the former case the composition is placed in a plastic condition over the article to be coated. and by shrinking firmly attaches itself thereto, and in the latter case the composition is compressed in suitably formed moulds or discs.

Lunar Nomenclature.-A committee of the Selenographical Society, which was appointed to consider the subject of lunar nomenclature, recom mends that some thirty of the names given to lunar objects within the last 25 years should be superseded by letters, and numbers attached to the names of larger and more important objects. It is understood that Professor Schmidt, of Athens, will carry out the recommendations of the committee in a catalogue of lunar objects which he is about to publish. Most of the names dropped are those of modern astronomers and friends of astronomers, some of whom are bot little known to fame. A similar self-denying ordi nance ought to be passed with regard to the names of objects on the planet Mars An Englishman recently travelling in the Rocky Mountains in com pany with an American astronomer severely criticised the American custom of advertising in large letters upon the rocks in picturesque places. "Well, it is bad," said the patriotic American; "but I guess we are not as bad as some of your people who have tried to advertise themselves on the planet Mars."

Athenæum.