CH. IV.-] 



MAGNIFICATION AND MICROMETRY. 



99 



The distance of 250 millimeters lias been chosen on the supposition that 

 it is the distance of most distinct vision for the normal human eye. 



Demonstrate the difference in magnification due to the distance at 

 which the image is projected, by raising the microscope so that the dis- 

 tance will be 350 millimeters, then lowering to 150 millimeters. 



In preparing drawings it is often of great convenience to make them 

 at a distance somewhat less or somewhat greater than the standard. In 

 such a case the magnification must be determined for the special dis- 

 tance. (See the next chapter. ) 



For discussions of the magnification of the microscope, see : Beale, 

 pp. 41, 355 ; Carpenter-Dallinger, pp. 26, 238; Nageli and Schwen- 

 dener, p. 176, ; Ranvier, p. 29; Robin, p. 126; Amer. Soc. Micrs., 

 1884, p. 183 ; 1889, p. 22 ; Amer. Jour. Arts and Sciences, 1890, p. 

 50; Jour. Roy. Micr, Soc, 1888, 1889. 



§ 154. Table of Magnifications and of the Valuations of the 

 Ocular Micrometer. — The following table should be filled out by each 

 student. In tcsing it for Micrometry and Drawing it is necessary to keep 

 clearly in mind the exact conditions under which the determinations were 

 niadc, and also the ways i?i which variation in magnification and the val- 

 uation of the ocular micrometer may be produced (§ 152, 153, 163, 166). 



