THE MICROSCOPE AND MICROSCOPIC METHODS 



17 



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 

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



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

 is % the size of the object, in propoirtion to their respective distances from the lens;, 

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



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. 



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

 that the opening angle of the beam proceeding 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 inverted 

 and actually much smaller than the object, the reduction (minificar 

 tion) being again measured by the quotient of the lens retina 

 distance divided by the object-lens distance. The longer the 

 antero-posterior diameter of the eye, the larger will be the retinal 



