Moreover, to complete the example, we must assume that the _existence_ of the watchmaker, and the fact of his having made the watch, are already admitted for other reasons. And we are only appealing to these marks of design to show that _when_ he made the watch, he must have known that it would be able to tell the time, and presumably made it for that purpose. And in this case the inference seems, if possible, to be still stronger. (2.) _The example of an eye._ We will next consider the _human eye_ as an example of natural organs showing marks of design. It is a well-known instance, but none the worse on that account. Now, in order to see anything clearly, it is necessary that an image or picture of it should be formed at the back of the eye, that is, on the _retina_ from whence the impression is communicated to the brain. And the eye is an instrument used for producing this picture, and in some respects very similar to a telescope. And its marks of design are abundant and overwhelming. To begin with, in both the eye and the telescope the rays of light have to be _refracted_, so as to produce a distinct image; and the lens, and humours in the eye, which effect this, somewhat resemble the lenses of a telescope. While the _different_ humours through which the rays pass, prevent them from being partly split up into different colours. The same difficulty had of course to be overcome in telescopes, and this does not seem to have been effected till it occurred to some one to imitate in glasses made from different materials the effect of the different humours in the eye.[1] [Footnote 1: Encyc. Brit., 9th edit., vol. xxiii., p. 137.] In the next place, the eye has to be suited to perceive objects at different _distances_, varying from inches to miles. In telescopes this would be done either by putting in another lens, or by some focussing arrangement. In the eye it is effected by slightly altering the _shape_ of the lens, making it more or less convex. A landscape of several miles is thus brought within a space of half an inch in diameter, though the objects it contains, at least the larger ones, are all preserved, and can each be distinguished in its size, shape, colour, and position. Yet the same eye that can do this can read a book at the distance of a few inches. Again, the eye has to be adapted to different _degrees of light_. This is effected by the _iris_, which is a kind of screen in the shape of a ring, capable of expanding or contracting so as to alter the size of the central hole or pupil, yet always retaining its circular form. Moreover, it is somehow or other self-adjusting; for if the light is too strong, the pupil at once contracts. It is needless to point out how useful such a contrivance would be in photography, and how much we should admire the skill of its inventor.