. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. latter. 76. By means of the iris, the cavity, (^,) in front of the crys- 5 60 SPECIAL SENSES. talline lens is divided into two compartments, called .he anterior and posterior chambers. The fluid which f, is tnesbchambers is a clear watery liquid, called the aqueous humorThe portion of the globe behind the lens, which is much thelargest, is filled by a g
. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. latter. 76. By means of the iris, the cavity, (^,) in front of the crys- 5 60 SPECIAL SENSES. talline lens is divided into two compartments, called .he anterior and posterior chambers. The fluid which f, is tnesbchambers is a clear watery liquid, called the aqueous humorThe portion of the globe behind the lens, which is much thelargest, is filled by a gelatinous liquid, perfectly transparent,like that of the chambers, but somewhat more dense. Thisis called the vitreous humor., (h.) 77. The object of this apparatus is to receive the rays oflight, which diverge from all points of bodies placed beforeit, and to bring them again to a point upon the retina. It isa well-known fact, that when a ray of light passes obliquelyfrom one medium to another of different density, it will berefracted or turned Dut of its course more o~ ^ss, accordingto the difference of this density, and the obliquity at whichthe ray strikes the surface. This may be illustrated by thefollowing figure, (Fig. 14.) A/ E. Fig. 14. The ray a c, which strikes the cornea A B perpendicularly,continues without deviation, until it reaches the bottom otthe eye at c. But the rays a m and ari, which strike the eyeobliquely, change their direction, and instead of proceedingonward to m g and n f/, take the direction 7n i and n f Astill further refraction, though less considerable, is occasionedby passing through the crystalline lens C D, and the vitreoushumor, so that the two rays, 7n i and nf., will at last meet ina point. This point is called \\q focus., (c,) and in distinctvision is always precisely at the retina, E F. 78. From this arrangement, the image found upon the OF SIGHT. 51 retina will be inverted. We may ourselves (.f thisby direct observation. The eye
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