136 



PHOTOMICROGRAPHY 



and Messrs. Zeiss now make an aplanatic condenser, Ic, 

 with a N.A. of -4, which is even smaller than la, and used 

 with small arc lights ; it is of shorter focus than 1 or la, 

 and is placed nearer the lamp than they are. The 

 collector projects an image of the radiant across the 

 surface of the second condenser which gives a convergent 

 beam covering the objective with an even disc of light. 

 The large condenser 3, shown in Fig. 44, is now discon- 

 tinued and replaced by No. 5, the smallest of the series. 



The object is carried on a stage, consisting of an iris 

 diaphragm with clips, and fitted with a screw movement 

 for final focussing. 



One of the newer aplanatic condensers can be used in 



Fig. 45 



DIAGRAM OF ILLUMINATION FOR SMALL ANASTIGMATS 



place of the two condensers, and the diagram, Fig. 45, 

 shows the method of illumination then adopted. -It is 

 drawn approximately to scale for a Watson-Conrady 

 condenser and a 2" lens, with a Nernst lamp about 5" from 

 the condenser. A convergent beam of light is projected 

 through the object, forming an image of the radiant across 

 the objective sufficiently large to fill its aperture. The 

 superiority of the corrected condensers has already been 

 referred to, and the fact pointed out that a small one can 

 concentrate a much more intense illumination over a small 

 area than a considerably larger simple lens. The object 

 is placed close to the condenser, and the diameter of the 

 latter must be sufficient to evenly illuminate the whole 

 surface to be photographed. There is no need for it to 

 be greater than that ; any excess of light is wasted, 

 or gives trouble. The diameter of an uncorrected con- 



