20 



VEGETABLE HISTOLOGY. 



their focal length at will so as to perceive objects at different 

 distances away. There are, however, several possible optical 

 defects in eyes, which may arise from various causes : 



1. The rays may be brought to a focus in front of the retina 

 instead of on it. Such eyes are called near-sighted, and may be 

 helped by the use of diverging lenses, which cause the rays to 

 become less divergent in the eye and thus to meet in a focus 

 farther back on the retina. 



2. The rays may be focused back of the retina. Such eyes 

 are called far-sighted, and may be helped by the use of con- 

 verging lenses, which act in a manner oposite to that stated for 

 diverging lenses in 1. 



3. The focus may be different for different sections of the 

 eye. If the dial of a clock be looked at an eye may see the 

 figures 2 and 8 clearly, but may not see the 5 and 11 sharply. 

 Such eyes are called astigmatic, and may be helped by the use 

 of cylindrical lenses. (Ames' Theory of Physics.) 



MEASURE OP MAGNIFICATION IN A SIMPLE MICROSCOPE. The 

 apparent magnitude of an object is the angle it subtends at the 

 eye of the observer. In the case of two objects seen at the same 

 distance, the ratio of the apparent diameters is the same as that 

 of their absolute magnitude. Hence, in a simple microscope 

 (also in a compound one), the magnification is equal to the 

 ratio of the apparent diameter of the image to that of the object, 



Fig. 20. 



both being at the distance of most distinct vision. But as the 

 apparent diameters are not easy to measure, a simpler method 

 is used which gives an approximate measurement (Fig. 20). 

 AB is an object and A'B' its image, formed at the distance of 

 distinct vision for the eye E. Since the eye is always very close 

 to the lens, the angles subtended by the object and image may 



A'OB' 



be taken as A'OB' and aOb, and the magnification = 



aOb 



This is approximately equal to 



A'B' A'B' 



, and bv similar tri- 

 ab AB 



A'B' DO (dist. of distinct vision) 12 to 14 inches 



mg ' AB ~RO (dist. of object from lens)~~ CO (focal length)' 

 since the object is very nearly at the focus. Hence, magnifica- 

 tion by convex lens = ratio of distance of distinct vision (say 



