138 MAGNIFICATION AND MICROMETRY [CH. IV 



For example suppose the long axis of a Necturus' red blood 

 corpuscle measures 9 mm. on the eikonometer scale. If the magni- 

 fication of the microscope is 200 as found above then the actual 

 length of the corpuscle is 9 mm. -1-200= 0.045 mm., or 45/1. (See 

 A. E. Wright, Jour. Roy. Micr. Soc., 1904, pp. 261, 279; Princi- 

 ples of Microscopy, pp. 145, 163.) 



D 



>! x ^ ^ 



" MICROSCOPE EIKOI.OMETER 



FIG. 122. Wright's Eikonometer for Magnification and Micrometry 

 (from Beck's Catalog.) 



A. Objective; B. Ocular; D. The object; E. Virtual image of the 

 microscope ; C. The Eikonometer placed over the ocular. The lens G, 

 produces a real image on the eikonometer scale at F. This scale and real 

 image are then viewed through the Ramsden ocular of 2j mm. equivalent 

 focus, H. 



\ 197. Remarks on Micrometry. In using adjustable objectives ($ 27, 

 114), the magnification of the objective varies with the position of the adjust- 

 ing collar, being greater when the adjustment is closed as for thick cover- 

 glasses than when open, as for thin ones. This variation in the magnification 

 of the objective produces a corresponding change in the magnification of the 

 entire microscope, and the ocular micrometer valuation therefore it is neces- 

 sary to determine the magnification and ocular micrometer valuation for each 

 position of the adjusting collar. 



While the principles of micrometry are simple, it is very difficult to get 

 the exact size of microscopic objects This is due to the lack of perfection 

 and, uniformity of micrometers, and the difficulty of determining the exact 

 limits of the object to be measured. Hence, all microscopic measurements 

 are only approximately correct, the error lessening with the increasing perfec- 

 tion of the apparatus and the skill of the observer. 



A difficulty when one is using high powers is the width of the lines of 

 the micrometer. If the micrometer is perfectly accurate half the width of 

 each line belongs to the contiguous spaces, hence one should measure the 

 image of the space from the centers of the lines bordering the space, or as 

 this is somewhat difficult in using the ocular micrometer, one may measure 



