184 DETERMINATION OF MAGNIFYING POWER. 



power is 50 diameters. If, again, each, of the divisions of the 

 1- 1000th inch Micrometer correspond to 6-10ths of an inch upon 

 the rule, the magnifying power is 600 diameters ; and if it corre- 

 spond 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, or, still better, by pro- 

 jecting 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 com- 

 passes 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 

 (§ 68) 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, although often employed to 

 excite popular admiration, or to attract customers, by those whose 

 interest is concerned in doing so. + 



* See Hendry 'On Amphipleura pellucida,' in "Quart. Journ. o 

 Microsc. Science," Vol. i. N.S. (1861), p. 87. 



t It may be well here to remark, that the designations given by 

 Opticians to their Objectives are often far from representing their real 

 focal length, as estimated by that of single lenses of equivalent magnify- 

 ing power (§ 111) ; a temptation to underrate them being afforded by the 

 consideration that if an Objective of a certain focus will show a Test- 

 object as well as another of higher focus, the former is to be preferred. 

 Thus it happens that what are sold as half -inch Objectives are often 

 more nearly 4-10ths ; and that what are sold as l-4ths are not un- 

 frequently more really l-5ths. 



Addendum. — Since this chapter has been put in type, Mr. Ross has 

 brought out an Achromatic Objective of four inches focus; which will 

 prove of great service to those who study large Opaque objects, whose 

 inequality of surface demands great focal depth or penetrating power. 



