the eye when immersed in water. They are not focused at the retina, so that the image formed on the retina is very indistinct, as it must be when so imperfectly focused. The continuous line shows how the rays of light are first deflected by the concave air lens ; this deflexion, in the case of a lens whose curves are such as stated above, being sufficient to make up for the loss of refraction by the extinction of the aqueous humour lens when it receives the rays of light through the identically refracting watery medium. 47. I thought it might be advantageous, or at all events agreeable, to be able to see distinctly when diving; so I constructed a pair of spectacles fitted with air lenses of the kind just described. It is obvious that spectacles fitted with air lenses would be much more convenient for diving than glass lenses. For whereas the glass lenses required for subaqueous purposes are of such very short focus in air (only three-eighths of an inch), that they would prevent all vision when the diver came to the surface, these air lenses would offer no impediment to perfect vision in the air, and so might continue to be worn with equal advantage both in air and water. I found, however, that the two sections of a glass globe which form the concave air lens, have in the air the effect of a very weak concave glass lens, such a lens as is used to correct the slightest degree of short sight. The cause of this is that the inner concave surface of the glass globe is a curve of smaller radius than the outer convex surface. Thus it is a concavo-convex lens, though of very small power. But when two such glasses are placed together the refraction they produce is appreciable, and somewhat impairs perfect vision to a non-myopic eye. 48. In order to counteract this slight refraction in the lens when used in the air, in place of having the glasses made of sections of a glass globe, I had them ground with surfaces of precisely the same curvature. By this means I obtained glasses for my air lenses which, having their surfaces of precisely the same curve, cause no deflexion of the rays of light. These lenses therefore restore perfect vision beneath the water, and offer no impediment to perfect sight in the air. 49. Vision below the water, if the latter is quite clear, is with lenses of this description quite perfect for both near and distant objects. Thus with them we can see to read the smallest type at the distance of a foot, while we can also see objects at many yards distance. Everything below the water is seen in its proper proportions, and without any distortion whatever. Thus the pattern of the porcelain tiles on the bottom and sides of a bath, as in many of the swim ming baths in London, anything lying upon the floor of the bath, such as coins, stones, pins, or other small articles, the bodies and limbs of other bathers, and any objects floating in the water, are all seen distinctly and accurately.* * In 1871 I published a pamphlet entitled 'Notes on the Dioptrics of Vision,' in which I gave a description of the air lenses for use under water. This pamphlet I sent to several scientific gentlemen, and among them to the late Sir John Herschel, through his publishers, Messrs. Longmans. A few days afterwards I received the following letter : "SIR, "COLLINGWOOD, Jan. 22, 1871. "I beg to acknowledge, with thanks, your paper, 'Notes on the Dioptrics of Vision.' "The idea of employing a double concave air lens in water, instead of the more obvious double convex glass one, is ingenious, as it ceases to be a lens as soon as it is out of the water, and (barring distortion from wet external surfaces) would not impede vision if used as spectacles out of water. Excuse me, however, if I remark that the way in which the principle of construction is stated in page 8, line 10, &c., had to myself, on a first rapid perusal, the effect of creating some degree of obscurity as to the actual disposition intended, which was only dissipated, as I read on, by the use of the term 'air lens' in page 15, line 8, when all became clear. "I have the honour to be, Sir, "Your obedient servant, "J. F. W. HERSCHEL. "P.S.-On reperusal I see the principle is all very clearly stated in page 5, line 10, et seq. But this, by turning over two leaves at once, I unluckily missed. "P.S.-M. Chossat long ago pointed out the non-sphericity of the crystalline lens, and showed that the curvature was that of an oblate spheroid, the central portion being less curved than the exterior. "Dr. DUDGEON." 50. The effect of perfect vision, such as these lenses afford, beneath the surface of the sea is curious and unexpected. If the sea is perfectly clear, as it is on many parts of the coast, when we dive, say from 6 to 10 feet in water of about 20 feet in depth, we see the bottom most distinctly; every stone, every shell, every leaf of seaweed that may be growing on the sand is distinctly visible over a considerable space. But on looking around us horizontally we are surprised at the gloom of the prospect. However bright the day may be above, we seem to be looking into the depths of an obscure cavern, wherein is "No light, but rather darkness visible." The weird void that seems to extend all around us, unless where it is broken by a fragment of floating seaweed, some air-bubbles hastening up to the daylight, or perhaps the white limbs of some distant fellowbather, seems unaccountable under a bright sunshine. If we turn on our back and look upwards, we shall find that the light seems not to penetrate all the surface, but only a limited portion of it. The surface of the sea is seldom so smooth as to enable us to note exactly what takes place, but if we will gently glide to the bottom of an ordinary swimming bath when the surface is undisturbed by other bathers, we shall see what remarkable effects are produced by the refractive power of the water on the rays of light coming to it from the air. 51. In the diagram (Fig. 17) I have endeavoured to represent what will be seen by an observer looking up FIG. 17. B E wards from the centre of the bottom of a small swimming bath. The knees of the bather B will appear to be at b; the middle of C's body will be elevated to c: the clock D will be raised to d; the wall of the room at E will be at e. The lamp F being directly above the observer's eye, the rays of light proceeding from it perpendicularly will suffer no refraction, consequently the lamp will be seen in its true place. All parts |