LOW-POWER PHOTOMICROGRAPHY 



137 



denser must be considerably larger than that of the object, 

 as the light passing through its outer zones must be cut 

 off by a diaphragm ; if utilised, the illumination will not be 

 even, and will fall off at the margin of the field, and show 

 as a distinct circle of under-exposure in the photomicro- 

 graph. An apparatus arranged for photography with 

 planars, and using a microscope, is shown in Fig. 46. The 

 substage condenser is seen in use in place of the auxiliary 



Fig. 46 



I'HOTOMICIIOGKAFIIIC APPARATUS ARRANGED FOR USE WITH 

 SMALL ANASTIGMATS 



condenser indicated previously. The back combination 

 of an Abbe or achromatic condenser may be of sufficient 

 diameter and focal length to illuminate an object photo- 

 graphed with a 2" lens, but a 3" usually requires one of the 

 special condensers mentioned on page 107. A double con- 

 vex lens of 2" diameter and 3" focus, which just fitted the 

 recess beneath the microscope stage, was used to photo- 

 graph the longitudinal section of the mouse, Plate 15. 

 A Nelson achromatic condenser or an ordinary bull's-eye 

 can also be used. The one chosen is racked up close 

 to the object, or placed as near to it as the stand will 



