PHOTO-MICROGRAPHY BY ELECTRIC LIGHT. 21? 



with very high powers, a circumstance of considerable 

 interest where motives of economy preclude the micro- 

 scopist from procuring this convenient instrument. 



The time of exposure required for the production of 

 pictures magnified 500 diameters or less was about a 

 second. With higher powers it increases, varying with 

 the management of the achromatic condenser. For 4000 

 diameters as much as twenty-five seconds is sometimes 

 required. 



In using the electric light it is necessary to render the 

 divergent pencil proceeding from the carbon points as 

 nearly parallel as possible, by means of the condenser 

 usually supplied with electric lamps for this purpose, and 

 then introduce into the parallel pencil, instead of a ground 

 glass, the very same condensing lens described above for 

 the process with solar light. The image is received 

 primarily for focussing on a cardboard screen, and the 



FIG. 197. 



remaining details do not differ from what has been 

 related above. The time of exposure does not exceed a 

 single second for 400 diameters, and the sharpness of the 

 pictures is excellent. For well-made tissue preparations, 

 however, the best work obtainable with the electric light is 

 so similar to the best attainable by sunlight, used as above 

 described, that it is scarcely necessary to take the trouble 



