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76 THE PHOTOGRAPHIC NEWS. [Feb. 14, 1862. to be the same as the sides of the camera, provided the lens be well shaded; it is only constructed so usually for the sake of the uniformity of its appearance. The whole of the inside of the shade should be coloured a dead black, or, better still, lined with black velvet, for this being outside the camera there is no reason to fear about dust. (To be continued.) PROFESSOR TYNDALL’S LECTURES ON LIGHT, Lecture in.—Dec. 31, 1861. Professor Tyndall commenced this lecture by making a few concluding observations on the refraction and reflection of light. He took a large piece of glass with parallel polished sides, and by means of a bundle of rays from the electric light, showed to the audience that when a beam of light fell upon it a portion of it was reflected, making the angle of incidence equal to the angle of reflection ; a por tion, however, of the light passed through, and might be found on the other side after having passed through the glass. The light, after having passed through the first sur face, and arriving at the lower surface of the glass, did not, however, all escape into the air; some was reflected back again, although the greater portion was refracted through the surface below. The portion of the light reflected upwards from the bottom surface was again split up, on arriving at the top surface, into a refracted ray, which passed out into the air, and into a reflected ray, which went downwards again to the bottom surface. Here, again, the same thing hap pened, and thus were obtained a series of reflections from the two internal surfaces of the piece of glass. This can readily be observed by any one: take a looking-glass, and, having placed a candle near it, look obliquely at its image in the glass. First of all there is perceived a tolerably bright image, being the one reflected from the silvered surface behind, and then follow a series of images caused by the re flections of the light from side to side becoming gradually fainter and fainter until they are invisible. Another matter of some importance was then referred to. It was stated, that where there was refraction there was reflection, and where there was no refraction there was no re flection. If any solid or liquid bodies were taken, it was no matter how different they were in substance or in weight, if the solid only bent the rays of light to the same degree as the liquid, it acted upon it just as the liquid itself, and became invisible when immersed in it, owing to the absence of refraction and reflection at the two bounding surfaces. If the eyeball of an ox be plunged into water it vanishes ; it appeal's like the water, although it is a totally different sub stance. The whiteness of foam and of snow—each consist ing really of perfectly transparent bodies—was shown to be due to the repeated reflections of light from particle to par ticle, occurring so often as to render the mixture perfectly opaque. To illustrate this property of transparent bodies a piece of bibulous paper was taken. This was explained to consist of a multitude of partially transparent fibres, but because they are mixed up with air so much light is reflected at each surface that the paper appears white and opaque. If, however, a substance be introduced into the pores of the paper, which has a somewhat similar refractive power on the light to the fibres themselves, the great reflection and loss of | light will be avoided, and the transparency of the paper will be powerfully augmented. Olive oil is such a substance, and the great increase in the transparency of the paper upon touching it with a drop of this liquid was beautifully shown by projecting an image of the paper on the screen by means of the electric light. It was then explained that the terms dense and rare, applied to a refracting medium, did not refer in any way to the weight of the body. A substance may be optically denser than another, though it be the lighter of the two. Thus : spirit of turpentine floats on water, and is therefore lighter, or, in ordinary language, less dense than water; but a ray of light in passing from turpentine to water is bent from the perpendicular, and in passing from water to turpentine it is bent towards the perpendicular. In optics, therefore, the densest body is that which refracts most. The important subject of the total reflection of light was then treated of. Supposing a ray of light, passing from a denser medium to a rarer, struck the common surface of both so obliquely, that on quitting the denser medium it was re fracted so as just to graze the surface, the angle between that ray and the perpendicular would be called the limiting angle, and for this reason—because no ray that strikes the surface at a larger angle than the limiting angle can get out of the denser medium. All such rays, on striking the sur face, are totally reflected, according to the law explained in the first lecture. The limiting angle then marks the limits of possible transmission from a denser to a rarer medium. This was explained to be the only case in which the reflection of light was total. It was then shown that a jet of water might be filled with light which could not escape in consequence of the law of total reflec tion. An electric lamp was placed behind an iron vessel connected with the water pipes of the building, so that water could enter it and issue forth in the form of a jet from a holo in the front, near the top of the vessel. At the back, oppo site the hole from which the vein of water issued, was a plate of glass, so that the lecturer was enabled to send a beam of light from the electric lamp through the glass, and straight through the vessel, issuing out of the hole in front in the form of a cone of light. The vessel was then filled with water until it kept running out of the front hole in the form of a jet: upon then turning on the light the beams of light which formerly passed through the hole in the form of a divergent cone, struck obliquely against the interior surface of the vein of water. The consequence was, that they could not get out, and were therefore actually washed down as if the light were a tangible substance, and reflected from side to side; but so obliquely, that they could not quit the liquid. In this way the whole of the jet of water was illuminated from top to bottom by the carrying down of the light which formerly passed straight through. When the Lecturer interposed coloured glasses in front of the lamp, the effect was most gorgeous, the vein of water shone with the most vivid colours, appearing alternately a bright blue, golden yellow, deep purple, and intense fiery crimson. The experiment is one of the most beautiful in the whole range of optics, and elicited universal admiration. The instrument known as the magic lantern was then described. It was shown to consist simply of two parts, one to illuminate the object, and the other to make a magnified image ot that object on the screen. It was was illustrated by placing a glass transparency in front of the electric lamp, and then arranging a single lens in front of this so as to project an image of the picture on the screen. The compound solar microscope was shown to be in effect the same thing as a magic lantern only much more refined. It consists of a little system of lenses, by means of which a very high magnifying power can be brought to bear upon any object placed between two slides and powerfully illumiated by other lenses close to the lamp. By means of an apparatus of this kind the singularly beautiful pheno mena of the formation of crystals from a saturated solution of sal-ammoniac was shown on the screen. At first a plain white disc was seen ; then the film appeared to move in one corner, and the “ atoms marched in time,” running together as if they were alive, weaving a crystalline web of such delicate beauty that nothing man ever formed could approach it. The human eye, that most wonderful optical instrument of all, was then described. This was shown to consist first of all of a substance just in front of the eye like a watch glass, called the cornea; it holds a little fluid called the aqueous humour, and behind that there is a little lump of jelly-like matter called the crystalline lens. Behind is the general mass of i . ■ ball ' Il 1 with what is called ti ■
