ON MEASUEING MICEOSCOPIC OBJECTS xxxi 



microscope in such a position that the scale can be seen dis- 

 tinctly on looking through the microscope. 



The stage micrometer is a slide on which a scale is en- 

 graved, divided to, say, hundredths and thousandths of a 

 millimeter. 



To measure a minute object, focus it under the micro- 

 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 sheet of paper laid on the 

 table at the side of the microscope is visible at the same 

 time as an object under the microscope, and if a scale be 

 drawn upon this coinciding with the apparent image of the 

 stage micrometer scale seen through the microscope, the 

 paper may be used to measure directly the size of an object 

 whose image is superimposed upon it. 



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. 



XI. ON PBEPAEING SKELETONS. 



Skeletons of rabbits, fowls, etc., may be prepared by mace- 

 ration or by boiling. Before macerating the bones should 

 have most of the flesh cut off ; they are then to be put into 

 cold water and left for several weeks for the remaining flesh 



