THE MICROSCOPE. 



133 



It is well to have three eye-pieces, having an equivalent 

 focal distance of 50, 35 and 25 mm. respectively. This cor- 

 responds to numbers 0, 2 and 4 of Leitz, and 2, li and 1 

 inch eye-pieces of this country. As a rule, the low eye- 

 piece should be used for work. ■ The higher eye-pieces may 

 be used, when it is desired to increase the magnification. It 

 should be remembered, however, that the eye-piece magni- 

 fies the image, and hence, any imperfections that may exist 

 in the image. On account of eye-piece magnification, the 

 field becomes darker and less distinct. It is preferable, 

 therefore, to obtain an increased magnification by changing 

 the objectives, rather than by the use of deep oculars. 

 When apochromatic objectives, are used, it is possible to 

 employ even the deepest compensation oculars. 



The Abhe condenser. — This is a most important accessory 

 to a microscope, and, indeed, it should not be wanting on 

 an instrument intended for bac- 

 teriological work. The usual 

 Abbe condenser is chromatic, 

 although for special purposes, as 

 in micro-photography, an achro- 

 matic condenser is employed. 

 The ordinary condenser is made 

 of two lenses, as shown in Fig. 

 19, and has a numerical aperture 

 of 1.20. By the introduction of 

 a third lens, the aperture is in- 

 creased to 1.40. 



The aim of the condenser is to illuminate the object as 

 much as possible. Without a condenser, given a plane 

 mirror, an object will be illuminated by a pencil of light 

 corresponding to its own diameter. This may be sufficient 

 for a low power, but when high powers are used, it is 

 evident that more light must be thrown on the object in 

 order to see distinctly. By means of the condenser, the 



Fig. 19. Cross-section of the Abbe 

 condenser, showing convergence of the 

 rays of light. 



