4Q 



MAGNIFICATION AND DRAWING. 



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 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. 



It may be remarked further that if spectacles are not used, a near- 

 sighted (myopic) person would obtain a somewhat greater, and a far- 

 sighted (presbyopic) person a somewhat less magnification for the same 

 instrument at the standard distance. This is because the eye of the 

 observer forms an integral optical part of the microscope at the time of 

 observation, and the equivalent focus of the myopic eye is less than 

 normal and that of the presbyopic eye greater (§ 7). 



For discussions of the magnification of the microscope, see : B., pp. 

 4 1 ) 355 ! £., PP- I 6i, 206 ; N. & S., p. 176 ; R., 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. 



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

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

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

 clearly in mind the exact conditions tinder which the determinations were 

 made, and also the ways in which variation in magnification and the val- 

 uation of the ocular micrometer may be produced (§§ 103, 104, 1 14, 1 16). 



MICROMETRY. 



§ 106. Micrometry is the determination of the size of objects by 

 the aid of a microscope. 



