140 A MANUAL OF BACTERIOLOGY 



"chromatic aberration." The apochromatic system of 

 objectives and eyepieces has these defects very perfectly 

 corrected by the use of special glass and fluorite, correction 

 being partly effected in the objective, and this is com- 

 pleted by combination with the special eyepieces. The 

 latter, termed " compensating oculars," are therefore 

 essential for perfect correction with apochromatic objec- 

 tives, but can also be used with ordinary lenses. For 

 photographic purposes apochromatic lenses are far superior 

 to achromatic ones. Apochromatic objectives are, how- 

 ever, expensive, and though advantageous are not really 

 necessary for ordinary bacteriological work. 



In consequence of certain optical principles, the 

 " diffraction " theory, for details of which the reader must 

 refer elsewhere, 1 it is useless to increase the magnifying 

 power of objectives beyond a certain point ; for, although 

 the object viewed appears larger, no more details of structure 

 can be made out. 



The use of the immersion system increases the " re- 

 solving power," or the amount of detail which can be 

 seen. Thus, if a number of fine equidistant parallel lines 

 be ruled on a glass plate, it is impossible to see with a dry 

 lens, using white light, more than about 90,000 lines to 

 the inch as isolated lines. If more are ruled they will 

 not appear, and practically nothing is visible. With a 

 water-immersion objective it is possible to see about 

 120,000 lines to the inch, and with an oil-immersion as 

 many as 146,000 lines to the inch, as separate lines a 

 clear gain in resolving power in the latter case of about 

 one half over a dry lens. 2 As it is necessary, in order to 

 see such fine structures as lines ruled 50,000 or more to 

 the inch must be, to have considerable amplification in 



1 See Carpenter on the Microscope, edited by Dallinger. (Churchill.) 



2 These figures refer to lenses having a numerical aperture of 1-0 

 (dry), 1-33 (water), and 1-4 (oil). 



