ON THE USE OF REAGENTS. xxix 



scope with, the eye-piece micrometer in position ; read off 

 the apparent size on the scale of the micrometer ; replace 

 the slide by the stage micrometer and read off upon that the 

 actual value of the noted number of divisions of the eye- 

 piece micrometer. 



The value of each division on the eye-piece micrometer 

 will obviously vary when the objective is changed or the 

 length of the microscope tube is altered. 



By means of a camera lucida a scale may be drawn upon 

 a sheet of paper laid at the side of the microscope, coincid- 

 ing with the apparent image of the stage micrometer seen 

 through the microscope, and this scale may then be used to 

 measure directly the size of an object seen at the same time 

 through the microscope. One such scale for each power of 

 the microscope may conveniently be drawn in the notebook 

 which is to receive the drawings, and the student will then 

 be able to determine the real size of any object drawn in the 

 book with the aid of the camera. 



Eye-pieces and stages are also constructed in which the 

 measurement is accomplished by means of a fine screw with 

 a divided head. To use the micrometer stage, the slide 

 bearing the object is laid upon it, one edge of the image is 

 made to coincide with a mark upon the eye-piece micrometer 

 or a slip of glass in the same position. The screw of the 

 stage is then turned till the opposite edge of the image 

 coincides with the same mark, the distance through which 

 the slide has been moved is then read off on the divided head 

 of the screw, and this distance is, of course, the diameter of 

 the object. 



IX. ON THE USE OF DISSOCIATING, HARDENING, 

 ND STAINING REAGENTS. 



Reagents are used for fixing, dissociating, hardening, and 

 staining specimens. Those required for general use are few 

 in number, and directions for their preparation will be found 

 in the Appendix. 



1. Fixing has already been explained. To obtain good 

 results it is necessary to employ either small animals or 



