PHOTO-MICROGRAPHY BY SUNLIGHT. 



215 



the shutter, and thence into the dark room. No ground 

 glass is used, but a lens mounted in a suitable tube is fixed 

 in the opening of the shutter through which the solar pencil 

 enters. This lens is an achromatic combination about 

 2 inches in transverse diameter and of about IO inches 

 focal length. It is placed at such a distance from the 

 achromatic condenser that the solar rays are brought to a 

 focus and begin again to diverge before they reach the 

 lowest glass of the achromatic condenser. 



FIG. 196. 



All trouble from the solar heat is thus completely 

 avoided. So successfully may this be done, that it is 

 possible to obtain light enough to give distinct vision and 

 admirable definition on the cardboard screen with 5000 

 diameters, while the heat is so slight that the drop of water 

 used with the immersion lens does not require renewal 

 oftener than about once in two hours. 



For anatomical preparations requiring for their display 

 from 200 to 500 diameters, the ^-inch objective may be 

 used without an eye-piece, obtaining the precise power 

 desired by variations in the distance of the sensitive plate 



