S6 



ELEMENTS OF APPLIED MICROSCOPY. 



be seen, and if, as more commonly occurs, the light from 

 the paper is the strongest, the object becomes almost 

 invisible. When the illumination is properly balanced, 

 both object and paper should be clearly seen, and the 

 point of a pencil on the paper may be made to trace the 

 outline of the object with accuracy. Obviously this 



Fig. 27. — CouKSE or Rays in the Camera. Lucida. (After Gage.) 



device, primarily intended as an aid to the making of 

 drawings of microscopic objects, may be used in microm- 

 etry. With a given arrangement of apparatus, the size 

 of the drawing on the paper will depend upon the size of 

 the object. By removing the latter and substituting a 

 stage micrometer whose image will be superposed upon 

 that of the drawing, the size of the original object may 

 be read off directly. 



