In this diagram L L is a convex lens of crown-glass, and l l a concave one of flint-glass. A convex lens will refract a ray of light (S) falling at F on it exactly in the same manner as the prism A B C, whose faces touch the two surfaces of the lens at the points where the ray enters, and quits. The ray S F, thus refracted by the lens L L, or prism A B C, would have formed a spectrum (P T) on a screen or wall, had there been no other lens.
Fig. 20.—Virtual Image formed by Convex Lens.
Formation of Virtual Images.—The normal eye possesses a considerable power of adjusting itself to form a distinct image of objects placed at varying distances; the nearer, within a certain limit, the larger it appears, and the more distinctly the details are brought out. When brought within a distance of two or three inches, the images become blurred or quite indistinct, and when brought closer to the eye, cannot be seen at all, and it simply obstructs the light. Now the utility of a convex lens, when interposed between the object and the eye, consists in reducing the divergence of the rays forming the several pencils which issue from it, and send images to the retina in a state of moderate divergence, that is, as if they had issued from an object beyond the nearest point of distinct vision, and so that a more clearly defined image may reach the sensitive membrane of the eye. But, not only is the course of the several rays in each pencil altered as regards the rest, but the course of the pencils themselves is changed, so that they enter the eye under an angle corresponding with that under which they would have arrived from a larger object situated at a greater distance, and thus the picture formed by any object corresponds in all respects with one which would have been made by the same object increased in its dimensions and viewed at the smallest ordinary distance of distinct vision. For instance, let an object A B ([Fig. 20]) be placed between a convex lens and its principal focus. Then the foci conjugate to the points A B are virtual, and their positions can be found by construction from the consideration that rays through A, B, parallel to the principal axis, will be refracted to F, the principal focus on the other side. The refracted rays, if produced backwards, must meet the secondary axis O A, O B in the required points. An eye placed on the other side of the lens will accordingly see a virtual image erect, magnified, and at a greater distance from the lens than the object. This is the principle of the simple microscope.
The Human Eye.
To gain a clear insight into the mode in which a single lens serves to magnify objects, it will be necessary to revert to the phenomena of ordinary vision. An eye free from any defect has a considerable power of adjusting itself to very considerable distances. One of the special functions of the eye is bringing the rays of light, by a series of dioptric mechanisms, to a perfect focus on its nervous sensitive layer, the retina. The eye in this respect has been compared to a photographic camera. But this is not quite correct. The retina is destined simply to receive the images furnished by the dioptric apparatus, and has no influence upon the formation of these images. The luminous rays are refracted by the dioptric apparatus; the images would be formed quite as well—indeed, even better in certain cases—if the retina were not there. The dioptric apparatus and its action are absolutely independent of the retina.
The same laws with regard to the passage of the rays of light into the human eye hold good, as those already enunciated in the previous pages. As to change of direction when rays are passing obliquely from a medium of low density to that of a higher density, i.e., it changes its course, and is bent towards the perpendicular. On leaving the denser for the rarer medium it is bent once more from the perpendicular. Again, by means of a convex lens, the rays of light from one source will be refracted so as to meet at a point termed the principal focus of vision.
In the eye there are several surfaces separating the different media where refraction takes place. The refractive index of the aqueous humour and the tears poured out by the lachrymal gland is almost equal to that of the cornea. We may, therefore, speak of the refracting surfaces as three, viz.: Anterior surface of cornea, anterior surface of lens, and posterior surface of lens; and also of the refracting media as three—the aqueous humour, the lens, and vitreous humour. These several bodies are so adapted in the normal eye that parallel rays falling on the cornea are converged to a focus at the most sensitive spot (the yellow spot, or fovea centralis) in the retina, a point representing to the principal focus of the eye. A line drawn from this point through the centre of the cornea is called the optic axis of the eye-ball.
Fig. 21.—Nerve and Stellate Cell Layer of Cornea,[6] stained by chloride of gold; magnified 300 diameters. a, Nerve cells. b, Stellate cells.