CH. 1 V\ JfAGNIFICATION AND MICROMETRY 



125 



triangle, of which it forms a base. The distance of 250 millimeters 

 has 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 distance will be 350 millimeters, then lowering to 150 milli- 

 meters. 



Fig. 114. Sectional 

 view of the Abbe Cam- 

 eya Lucida to show that 

 in measuring the stand- 

 ard distance of 250 mill- 

 imeters, one must meas- 

 ure along the axis from 

 the point P, at the left 

 of the prism, to the mir- 

 ror, and from the mir- 

 ror to the drawing sur- 

 face. For a full ex- 

 planation of this camera 

 lucida, see next chapter. 



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 distance. (See the next chapter, § 207. ) 



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

 pp. 41, 355; Carpenter-Dallinger, p. 288; Nageli and Schwendener, 

 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; 1904, pp. 261, 279; A. E. Wright, 

 Practical Microscopy, pp. 129, 145, 163. 



§ 179. Table of Magnification and of the Valuations of 



