466 THE POPULAR SCIENCE MONTHLY. 



essential design of this tissue is to intercept the scattered rays. But, 

 apart from its office of conducting nourishment to the eye, and secret- 

 ing the humors by means of its numerous blood-vessels, the choroid 

 has a second optical design to fulfil, which now brings us to the char- 

 acteristic signs of the eye. 



As you will perceive from Fig. 4, which gives a section of an eye 

 from life, the choroid, after having accompanied the sclerotic to the 

 edge of the cornea, goes on expanding anteriorly, and from henceforth 

 bears another name, that of the iris, or, as we might say, the rainbow 

 tissue. As this process, which likewise contains a quantity of pigment, 

 lies behind the transparent cornea, it can be observed in all its minu- 

 tiae ; and, on account of the rayed arrangement adopted by its fibres, 

 it is frequently called the eyeball, or star. The iris is broken in the 

 centre by an opening to which we apply the term pupil, being the 

 visual aperture. It usually seems to be black. 



The presence of the iris greatly diminishes the extent of surface 

 designed for the reception of light ; the whole pencil of rays that falls 

 on the cornea, as supposed, in Fig. 3, not reaching the retina, but only 

 as shown in Fig. 4, on that section of it which enters the pupil. 

 Though much of the volume of light is thereby lost, the restriction is 

 highly beneficial, by sharpening the image on the retina ; for the re- 

 fraction of the rays is much more equal in the centre than toward the 

 margin. 



The iris, however, has a still more important function to perform. 

 It regulates the entrance of the light, being furnished with a muscular 

 apparatus (ciliary muscle), which provides that in strong light the 

 pupil contracts, and in duller light expands. Thus the iris plays the 

 part of a so-called movable diaphragm, a common appliance in optical 

 instruments, used to dull the light for the purpose of seeing better. 

 You cannot but have observed this play of the pupil, and how it ac- 

 commodates itself to the volume of light ; nor can you be ignorant 

 that the iris with its varied coloring from light blue to deepest brown 

 is what we know as the color of the eye. It is perhaps less well known 

 to you, however, that the peculiar pigment required for the darker 

 colors of the iris comes only as we advance in life, and that, therefore, 

 we all commence our earthly course with blue eyes ; a fact already 

 known to Aristotle. 



The crystalline lens, which is held fast in its place by a very fine 

 tissue, as shown in Fig. 4, from the curve of its surface, and its strong 

 power of refraction, plays an important part by conducting the col- 

 lected light to the picture on the retina. It has, however, another and 

 extremely important design, which must here be carefully considered. 



The requirements made on an optical instrument depending on 

 lenticular effect, are different according as it is expected to project 

 images of nearer or more distant objects. The light with very diver- 

 gent rays, and proceeding from near objects, is collected to a picture 





