MEMOIR XXV.] MICROSCOPIC PHOTOGRAPHY. 339 



But these difficulties may be completely overcome by 

 causing the illuminating beam to pass through a solu- 

 tion of sulphate of copper and ammonia. This transmits 

 all the radiations that affect a photographic surface, but 

 absorbs all others. With the absorbed rays the heat so 

 nearly disappears that the most delicate preparation may 

 be left at the focal point for any length of time without 

 risk. 



The solution of sulphate of copper and ammonia also 

 enables us to ascertain the focal point with precision. 

 On receiving the image on the ground glass of a camera, 

 it is, of course, of a blue color, and when brought to a 

 sharp focus its photograph will be equally sharp. 



The best results are obtained when the blue image 

 of the sun, formed by the condenser, coincides with the 

 object. 



In Fig. 60, a a is a heliostat mirror reflecting the sun's 



Fig. CO. 



rays horizontally ; c a condensing lens three inches in 

 aperture ; there are adapted to it different diaphragms, 

 b, to regulate the quantity of light used, since it is not 

 well to have too bright an image on the photographic 

 plate ; d is a glass cell, formed by cutting a circular hole 

 three inches in diameter in a glass plate a quarter of an 

 inch thick ; on each face of this plate a flat piece of glass 

 is laid, thus forming a receptacle in which the sulphate 

 of copper and ammonia solution may be placed. It 

 holds the liquid without risk of leakage ; e is the object- 

 stage of the microscope,/; g, the camera with its ground 

 glass or sensitive plate. 



