OP TIC A L INSTR UMENTS. 



423 



(a.) One part of the box, B, slides within the other part, C. A 

 ground-glass plate is placed in the frame at E, which is adjusted so 

 that a well-defined, inverted image of 

 the object in front of A is projected 

 upon the glass plate. This adjust- 

 ment, or "focussing," is completed 

 by moving the lens and its tube by 

 the toothed wheel at D. When the 

 " focussing" is satisfactory, A is cov- 

 ered with a black cloth, the ground- p IG> 

 glass plate replaced by a chemically- 

 prepared sensitive plate, the cloth removed, and the image pro- 

 jected thereon. The light works certain chemical changes where 

 it falls upon this plate, and thus a more lasting image is produced. 

 The preliminary and subsequent processes necessarily involved in 

 photography cannot be considered here; they belong rather to 

 chemistry. 



657. The Human Eye. This most admirable of 

 all optical instruments is a nearly spherical ball, capable of 

 being turned considerably in its socket. The outer coat, S, 

 is firm, and, excepting in front, is opaque. It is called the 

 " white of the eye," or the sclerotic coat. Its transparent 

 part in front, C, is called the cornea. The convexity of the 

 cornea is greater than that of the rest of the eyeball. 

 Behind the cornea is an annular diaphragm, /, called the 

 iris. It is colored and opaque ; the circular window in its 



centre is called the pupil. 

 The color of the iris consti- 

 tutes the color of the eye. 

 Back of the pupil is the 

 crystalline lens, L, built of 

 concentric shells of varying 

 density. Its shape is shown 

 in the figure. This lens 

 divides the eye into two 

 chambers, the anterior 



