THE MICROSCOPE AND MICROSCOPIC METHODS 17 



aperture-image distance divided by object-aperture distance; also 

 that the sharpness of outline of the image increases and the 

 brilliancy diminishes as the size of the aperture is decreased. 



If the simple aperture be replaced by a convex lens and the 

 object and the screen be set at the conjugate foci of the lens, it 



FIG. 2. Image formation by a single lens. Note that the image, at the right, 

 is | the size of the object, in proportion to their respective distances from the lens; 

 the opening angle being f the size of the closing angle. 



will be seen that magnification is again the quotient of the aper- 

 ture-image distance divided by the object-aperture distance. 

 The sharpness of outline, however, depends now upon the quality 

 of the lens and the accurate adjustment of the distance, and 

 brilliancy is not seriously impaired in attaining definition. 



FIG. 3. Image formation by two lenses in series without magnification. Note 

 that the opening angle of the beam preceding from the object, at the left, is equal to 

 the closing angle of the beam forming the image at the right. 



Image formation in the human eye is an example of the work- 

 ing of the lens-armed aperture. The rays of light are brought 

 to a focus on the retina and the image produced here is in- 

 verted and actually much smaller than the object, the reduction 

 (minification) being again measured by the quotient of the lens- 



