136 Animal Micrology 



at which the image is formed may readily be determined. In 

 focusing a photographic camera, for example, the image comes 

 sharply into view on the ground-glass plate at the back of the 

 camera when the plate is brought into the plane in which these 

 rays through the focus and the optical center intersect beyond 

 the lens. It will be observed from the figure that the image is 

 reversed. The size of the image diminishes as the object lies 

 farther beyond F. 



In case the object lies between the lens and the principal focus, 

 as in Fig. 47, parallel rays from the object would converge to 

 meet at the conjugate focus F', and an eye at this point would 

 see the image projected and enlarged without being reversed. 

 The plane in which the image is formed is determined by finding 

 the points of intersection of the secondary axis through points of 

 the object with the imaginary elongation of the refracted rays as 

 shown in the figure. The image is magnified because the observer 

 judges of the size of an object by the visual angle which it sub- 

 tends. The greater the convexity of the lens, the shorter the 

 focus, and also, since the rays are bent more, the greater the 

 magnification. 



The Simple Microscope. The simple microscope (the ordinary 



so-called magnifiers, etc.) operates upon this principle; the image 



of an object is projected and enlarged but not inverted (Fig. 47). 



The question arises as to why there is a best distance to hold 



the simple microscope from an object. Why will not any point 



answer so long as it is within 

 the focal point? As a matter of 

 fact, the object may be placed 

 at any point within the focus, 

 and it will be found that the 

 nearer it is brought to the lens 

 the less it is magnified. There 

 is one most favorable point for 



observation, however, which is neither at the point of highest nor 

 of lowest magnification, but an intermediate point, where the lens 

 is freest from chromatic and spherical aberrations. 



