MICROSCOPY 



97 



that a bright, but small image of the object exists within a few centimeters of 

 the objective. About 10 cm away from the objective, and in line with the 

 object, is the "eye-piece," or condenser, another convex lens with very short 

 focal length, which throws an image of the objective's image on the retina if 

 held about 2 cm away. 



Figure 4-10b(ii). Absorption of X Rays by Tissues (Continued). Ab- 

 domen with Iodine Metabolized into the Kidneys. Note the difference be- 

 tween the normal calyces of the kidney (white "horse," upper left) and 

 the defective one (upper right). High contrast film. (Courtesy of A. F. 

 Crook, Ontario Cancer Foundation.) 



Magnifications up to more than lOOOx are possible with the best instru- 

 ments. The preparation of the lenses is the critical thing, for it is difficult 

 and costly to grind a large lens which will not be astigmatic. If the lenses 

 are perfect, the limit of resolution (the smallest distance by which two ob- 

 jects can be separated and still be differentiated) is determined only by the 

 wavelength of the light and the size of the aperture which admits the light. 



