tance, is unable to distinguish an object five inches off; and, if, against all rule, it see sharply at five inches off, this benefit is counteracted by the inconvenience of seeing distant objects with great indistinctness, The gradual hardening of the lens is accompanied by a decrease of power of adjusting its focus, and is even subject to such small individual fluctuations that, by an exact calculation of its play,[1] we may sometimes arrive at the most indiscreet conclusions respecting the age.
Should the lenticular elasticity no longer admit of a sufficient scope for refraction, we must then either adjust the distance of the objects, as we see a far-sighted individual do, by holding the book proportionably farther off, or we must afford the eye assistance by accommodating it with movable auxiliary lenses, spectacles, which replace the lost power of adjusting forms to the natural eye. This power of accommodating the focus disappears beyond recall if the lens has sustained an injury, or if we remove it entirely, from its having grown turbid. This takes place in the operation for cataract, which is a dimming of the sight from a thickening of the lens.
The lens, however, is not altogether free from optical irregularities, as when the focus of the eye has not been perfectly adjusted for seeing in the distance. Those of you who are short-sighted, on looking at a distant street-lamp, perceive, instead of the clearly-defined image, an irregular circle of light, and you will at the same time observe within that circle a number of peculiar rays and dots, which are nothing but irregularities in the lens, i. e., the reaction of those irregularities on the retina. Even an eye whose sight is quite normal, makes an analogous observation if it directs its gaze to a very fine point of light, as, for example, to a star. Both the star and the atmosphere are equally innocent of the small beams that radiate from it; they are the rays of our own lens which we transplant to heaven. So little aware are we of what takes place in the depth of our sensory organs, or in the immeasurable distances of the universe.
The spaces between the lens and the cornea, as also between the lens and the retina, are filled with a liquid medium, called humors; the latter, which constitutes by far the largest chamber of the eye, is filled with a gelatinous substance called the vitreous humor. This medium likewise contributes essentially to the concentration of the rays of light, as, lying between two curved partitions, they exercise a similar influence as the lens.
The vitreous humor, optically speaking, is, however, not pure; small granular or wavy forms, which all of you at times have seen hovering within your field of view, and which pursues so many hypochondriacal persons on their summer trip to a watering-place, are occasioned by shadows thrown on the retina by a partial, delicate opaqueness in the vitreous humor. Those bodies are so light as only to be
- ↑ The diminution of the play does not commence in the second period of our lives, but, as Donders has proved, in a regular manner from childhood onward.
