88 THE USE OF THE MICROSCOPE 



which is equal to or somewhat less than the object-field. 

 The back of the objective is then observed through a small 

 circular opening centered in a cap over the top of the micro- 

 scope tube, the eyepiece being removed; or by a 15-times 

 magnifying lens centered over the eyepiece. The solid 

 light circle on the back of the objective, when set at its 

 maximum by focusing the condenser, represents by its 



Wi+h'in Focus A+ Focus Beyond Focus 



ooo 



Over 

 Correction 



Correcfion 



Under 

 Corrcc+i on 



Fig. 19. — Diagram of the back of a high-power, oil-immersion objective used 

 with an aplanatic achromatic condenser corrected for water immersion. In 

 the upper row, the slide is too thick, or the lamp is too far away. On raising 

 the condenser, the disc of light merely gets smaller; but on lowering the con- 

 denser, a marginal ring of light appears. When the condenser is focused, its 

 aperture is seen to be lessened. In the middle line with adjustments correct no 

 marginal ring appears, whether the condenser is raised or lowered. The aperture 

 is at a maximum at the focus. In the lower line are shown the effects of too 

 thin a slide or too close proximity to the lamp. Here, on raising the condenser, 

 an outer ring appears, which is not the case on lowering it. The aperture at 

 the focus is also found to be lessened. This is the ring test. 



radius the used aperture of the condenser as compared 

 with the radius of the back lens, which represents the 

 aperture of the objective. (The condenser should have 

 been centered, if necessary.) Thus the maximum con- 

 denser aperture can be found. 



2. The edges of the image of the small diaphragm on 

 the source of light are observed when focused, along with 

 the object, by different objectives at different condenser 

 apertures. The focus of the condenser should not require 

 to be changed much, for the different apertures. If it 

 does, there is spherical aberration. 



