Графични страници
PDF файл

may be projected from the same opaque body, and the shadow formed in the light of one of the candles be lighted by that of the other. These shadows, as is known, will both be yellowish. Let one of the lights now be coloured red, by making it pass through a plate of glass of that colour; the shadow coming from the interception of the other light, will immediately assume a red tint, (objective colouring); but, at the very instant, the other shadow, which is only shone upon by the pure light of the other candle, will become green, (subjective colouring, produced in the organ of the observer itself, from the defect of the perception of the red ray); and vice versa, if one of the shadows is objectively and directly coloured green, the other will be subjectively coloured red.

Mr Trechsel here remarks, that the phenomenon observed by several members of the Helvetic Society of Natural Science, in a chapel near Soleure, appears to be of an analogous nature to the subjective coloration of shadows, in particular to the phenomenon last cited, and to that of the colouring of the bottom of the camera obscura by the light of the day. It will not be useless to recall here the description of this phenomenon, such as we have already given it in our account of the tenth session of the Helvetic Society *. All the panes of the windows of the chapel, without exception, are of pale yellow glass; the frames of these windows, which are of iron, are perforated here and there with small holes of about a line in diameter ; the light which penetrates by these holes, is of the most beautiful blue, even when through them the view is carried upon perfectly white clouds. The same effect is also produced when one of the windows is opened, and the slit thus formed is blue until the opening attains a certain width. We had explained this phenomenon, as probably arising from the psychological effect of contrast. Now this effect, which in general cannot be contested, may be owing to the momentary paralysation in the organ of the faculty of perceiving one of the partial sensations which compose the total impression.

Mr Trechsel concludes his memoir with the following brief review :

[merged small][ocr errors]

“ 1. Coloured shadows may be distinguished into objective and subjective.

“ 2. The former owe their colouring to the light which arrives at them either directly or by reflection; they are not therefore total shadows, but are rather, to use the scientific term, penumbre.

“ 3. The shadows whose colouring is subjective, are the effect of a particular disposition of our organ, which, when it is fatigued by the impression of a single colour, no longer perceives that ray in a fasciculus of white light; so that the complementary ray predominates and communicates its tint to the shadow projected in the primitive light. 66 4. So far as we have been able to observe, the eye

follows in this process the scale of Newton. If the corresponding colours are not always exactly complementary, it must be attributed to the difficulty of obtaining artificial tints so pure as those of the solar spectrum.

“ 5. There follows from this, that the colouring of shadows is impossible, if there be no other light than that by the interception of which the shadow is formed. The presence of a light coming from another part, for example, from the sky, or the clouds, is an indispensable condition to the formation of coloured shadows.

“ 6. Lastly, the shadow is not necessary to make the complementary colours appear. A small quantity of white light, put in prominent contrast with a large mass of coloured light, assumes, in certain circumstances which we cannot well determine, the complementary tint corresponding to the colour of this latter light."

Mr Trechsel's explanation appears to us satisfactory; it introduces, it is true, two causes for a phenomenon which has usually been considered as one; but this is not the first case where a careful analysis has obliged us to admit several agents in an effect single in appearance.

Without doubt, in the number of the very varied experiments which may be made on the subject of coloured shadows, there will still present themselves many details which will not be immediately explained ; but it is probable that their origin will be found in the peculiar circumstances of

[blocks in formation]

1. Skull of the Andaman Hog. 3

the Little Andaman Island. 3. Hut Fishing Net. 5 Conical B

these experiments, and in the state of the bodies employed by the philosopher in his observations.

Bibliotheque Universelle, May 1826.

Notice regarding the Little Andaman Island, Buy of Bengal.

Communicated by Cornet J. E. ALEXANDER, H. M. 13th

Light Dragoons. With a Plate. Os the 12th November 1825, the Honourable Company's transport, Earl Kellie, having on board 4 companies of H. M. 45th regiment, and 100 Madras pioneers, destined for foreign service in the Birman Empire, steering a S.E. course, hove in sight of the Little Andaman Island, in latitude 10° 45' N., and longitude 92° 12 E., bearing from E. to E.S E., and distant 4 leagues. At 10 A. M. bore up, and stood in for the land to get a supply of water, our stock of which was almost exhausted, from the unusual length of the voyage, occasioned by the baffling winds we experienced in the middle of the Bay. At 11 saw a brig, hull down, bearing N. W., steering to the S. E. At 12 sounded on a coral reef 8, 9, 10, 12, and 16 fathoms, patches, the bottom seen plainly under the ship, with numerous sharks following the vessel, one of which we succeeded in killing. Observed a murcena ophis or sea-serpent ; length about 3 feet; back brownish, belly white, tail rounded, a row of black spots along the sides, and without the caudal fin. A monstrous fish likewise made its appearance near the vessel, seemingly of the genus Raia or ray; its length about 20 or 25 feet, very broad, and colour of back reddish. A very heavy swell on the bank ; steering from N.N.E. to N.W. to haul off it, ran a distance of 4 knots, when the water deepened to 20 fathoms. When on the coral shoal, in 8 fathoms water, the extremes of the island from E. ; W. to N.N.E., and a small bay in the centre of the island ;-at 11 P. M. saw a light on shore ; brought up off the N.W. point, and anchored in 8% fathoms, at 2 miles from shore.

13th November.–At daylight proceeded in one of the cutters along with the chief mate in search of water. On approaching the shore, observed a woman and child on the beach, who, upon perceiving the boat, ran into the jungle; they appeared to

« ПредишнаНапред »