ENLARGING 



231 



ment than from a large negative. An example of a high- 

 power photomicrograph made by enlargement is given 

 on Plate 43, where a portion of a photomicrograph of 

 Amphipleura pellucida, the original magnification of which 

 is x 1250, has been further enlarged, giving a magnification 

 of 3750 times. This should be compared with the photo- 

 micrograph of Pleurosigma angulatum (Plate 42), in 

 which the original magnification is 5000 diameters. 



Many of the photomicrographic apparatus already 

 described can be adapted for enlarging, and Fig. 69 shows 



Fig. 69 



OPTICAL BENCH ARRANGED FOR ENLARGING OK PROJECTION 



Beck's large optical bench arranged for that purpose. 

 The diagram of the light rays indicates that the nega- 

 tive is placed close to a condenser whose diameter is 

 sufficient to cover the whole of the part to be enlarged. 

 4" or 4J" condensers are usually large enough for quarter- 

 plate negatives, when the whole surface is not required. 

 The second condenser throwing a parallel beam into the 

 converging lens is not essential, though advantageous ; 

 a single one can be used, focussing a convergent beam 

 over the objective. The lamp must be carefully centred 

 until a uniform disc of light is projected on a screen, 



