174: THE MICROSCOPE AND ITS REVELATIONS. 



If, again, each of the divisions of the l-1000th inch Micrometer corre- 

 sponds to 0.6 of an inch upon the rule, the magnifying power is 600 dia- 

 meters; and if it corresponds to 1.2 inches, the magnifying power is 

 1200 diameters. In this mode of measurement, the estimate of parts of 

 tenths on the rule can only be made by guess; but greater accuracy may 

 be obtained by the use of the Diagonal scale (Fig. 67), or still better, by 

 projecting the Micrometer-scale with the Camera Lucida at the distance 

 of ten inches from the eye, marking the intervals on paper, taking an 

 average of these, and repeating this with the compasses ten times along 

 the inch-scale. Thus, if the space given by one of the divisions of the 

 l-1000th-inch Micrometer, repeated ten times along the rule, amounts 

 to 6 inches and 2 tenths, the value of each division will be .625 of an 

 inch, and the magnifying power 625. It is very important, whenever a 

 high degree of accuracy is aimed at in Micrometry, to bear in mind the 

 caution already given ( 91) in regard to the difference in magnifying 

 power produced in the adjustment of the Objective to the thickness of 

 the glass that covers the object. The superficial Magnifying power is of 

 course estimated by squaring the linear; but this is a mode of statement 

 never adopted by Scientific observers. 



