154 A MANUAL OF BACTERIOLOGY 



tion, for no more will be seen unless the resolving power 

 can also be increased. So that the old idea of lenses of 

 -$ in., etc., is now obsolete. The immersion system 

 enables rays to be taken up by the objective, which with 

 a dry system could not be. This is illustrated diagram- 

 matically in Fig. 25, which shows the course of two light 

 rays. One, Yyt, with a dry lens would be refracted on 

 emerging from the cover-glass into air, pursue a direction 

 tw and would not be taken up by the lens. The other, 

 efk, would be totally reflected by the cover-glass, would 

 then pursue a reverse direction, fcP, and similarly would 

 not be taken up by a dry lens. If, however, the lens be 

 an immersion one, with oil between cover-glass and lens, 

 the refraction of the one ray and the reflection of the other 

 do not take place, the two rays pursue courses tv and 

 kv' respectively, and are taken up by the lens. The use 

 of the oil between lens and cover-glass practically converts 

 slide, balsam mount, cover-glass, oil and front lens of the 

 objective into a homogeneous layer, for they all have 

 approximately the same refractive index, and a light 

 ray, therefore, travels in a straight line through them. 



The construction of the two systems is different, so 

 that a dry lens can only be used dry, and an immersion 

 one immersed. 



The maximum resolving power of a dry lens (N.A. = 1-0) 

 using oblique illumination with ordinary white light is 

 about 90,000 lines to the inch, of a water -immersion lens 

 (N.A. 1-2) about 110,000 lines to the inch, and of an oil- 

 immersion lens (N.A. 1-4) about 127,000 lines to the 

 inch. Resolution with the latter, by the use of monochro- 

 matic short wave-length light, may be increased to about 

 146,000 lines to the inch, and by the use of special high- 

 refractive mounting media, to about 160,000 lines to the 

 inch, but this is the limit. That is to say, an object less 

 than about 0'16 JJL cannot be seen visually with the 



