418 APPENDIX. 



PLATE 8. The Printer, publishers, and Author may justly feel proud 

 of this page, produced in four colors from electrotyped wood-cuts. It is a 

 very difficult thing to register or adjust four colors as accurately as the 

 necessities of these illustrations require, and as perfectly as has been done 

 here. Better work of this kind has never been accomplished by any printer, 

 yet it has not been possible to have every copy exact. The entire consti- 

 tution of light, and the physical action of the eye upon it, is perfectly and 

 beautifully illustrated by the several figures. 



Fig. 1 represents light shining from one point of each of three candle 

 flames through an opening in the cornea of an eye without any refracting 

 media. Thus, the light from the three points is spread over a great ex- 

 tent of surface in the back part of the eye, and some from one point falls 

 upon the same surface as that from another point, preventing the possibil- 

 ity of distinctness of vision. 



Fig. 2 represents the light as in 1, acted upon by the media of the eye 

 and refracted upon corresponding points or foci, r, y, b, by which sensa- 

 tions may be distinct and numerous, and perceptions of directions accurate. 



Fig. 3 illustrates long sight, the media partially refracting the light. 



Fig. 4 illustrates short sight, the light being refracted too much. It 

 will be noticed that the effects upon the nerves in the back part of the eye 

 are the same in 3 and 4. The nerves do not terminate as represented in 

 these figures ; they, however, induce a correct idea of the method of seeing. 



Fig. 5 represents two rays of light acting on the same nerve, b d, in 

 which case a single sensation (of purple, when red and blue act) will be 

 caused. 



Fig. 6 represents two similar rays acting on two nerves, ac,db; in 

 which case two sensations will be caused. 



Fig. 7 illustrates white light, 10, shining through a small orifice into a 

 dark room, and passing through a prism, p, by which it is separated into 

 three kinds, red, yellow, blue. The separation does not take place thus 

 in the prism, as red and yellow will be blended at their margins, producing 

 orange, and yellow and t>lue producing green, etc., exhibiting all the colors 

 of the rainbow. The idea impressed by this figure, that there are three 

 kinds of light, is correct; and 



Fig. 8 more perfectly illustrates the proportions of the natural colors 

 that acting together produce white, and, blended or acting consecutively, 

 most highly please, namely, three parts red, five of yellow, and eight of 

 blue, making white sixteen. The figures 3, 5, 8, 16, may represent those 

 colors, when 13 will stand for green, 11 for purple, and 8 for orange, etc., 

 the proportions in which they must be combined to please most perfectly. 



Fig. 9 represents an eye, 2, looking through a hole in eye 1. The 

 light from I passes upward into eye 1, and comes downward into eye 2, 

 upon the lower part of which it will act, and the direction from which it 

 has come will appear to one eye to be the reverse of what it will seem to 

 be to the other. This figure explains all the peculiarities of inverted images. 



Figs. 10 to 17 represent by the "black lines so many different objects 

 with white light falling upon them, and absorbed, as in 10, causing the 

 object to appear black, or reflected as in 11, causing it to appear white; or 

 reflected in part, Fi<rs. 12 to 17, causing the objects to appear red, green, etc. 



