190 METHODS OF MICROSCOPICAL RESEARCH. 



an ordinary ground-glass screen, with cover glasses 

 cemented on to the ground side here and there, so 

 that at these places the screen becomes almost as 

 clear as plain glass, and parts of the image can be 

 examined through them with an ordinary focussing 

 lens. When the apparatus is used without the 

 eye-piece, focussing is effected by turning a rod 

 running underneath the camera, and actuating a 

 roller round which, and also round a groove in 

 the fine adjustment screw above, an elastic band 

 passes. 



The possible non-coincidence of the visual and 

 actinic foci of the object glasses is a point that 

 must be borne in mind, and each objective must be 

 tested to ascertain to what extent, if at all, this 

 non-coincidence occurs. This source of error is, 

 however, usually eliminated by interposing, in the 

 path of the beam between the two condensers when 

 focussing, a glass cell with parallel sides, containing 

 a solution of cuperic ammonic- sulphate 1 . This will 

 cut off all the rays of low refrangibility, and in nine 

 object glasses out of ten, will be found to supersede 

 any other correction, at all events, when paraffin is 

 the illuminant. "With the electric arc or other light 

 in which the ultraviolet rays largely predominate, 

 the case is different. The mode of testing for this 

 want of coincidence, and ascertaining its amount, 

 is to interpose the "copper" cell and obtain the 

 best possible visual focus of some object with hard 

 and sharp lines, with the glass to be tested. The 



i Prepared by adding ammonia to an aqueous solution of 

 sulphate of copper, until the precipitate first formed is re- 

 dissolved. 



