-ON THE USE OF REAGENTS. xxix 



The value of each division on the eye-pieee 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 l§,id at the side of the microscope, coinciding 

 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. Oiie 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 OP HARDENING AND STAINING REAGENTS. 



Eeagents are used for fixing, 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 

 small portions of large ones. Use a relatively large bulk of 

 the fixing fluid, and wash for several hours in water or alcohol. 



2. Hardening is necessary in the case of soft animals or 

 of tissues to be out into slices. Grades of alcohol 50 per cent., 

 70 per cent., and 90 per cent, are successively employed after 



