DETERMINATION OP MAGNIFYING POWER. 199 



be in the best possible position in regard to the light, whereas, 

 in the case of other finely-lined tests, it is only when the most 

 fayorable position has been attained, perhaps after tedious and 

 troublesome trials, that the markings are displayed.' 



103. Determination of Magnifying Power. — The last subject to 

 be here adverted to, is the mode of estimating the magnifying 

 power of Microscopes, or, in other words, the number of times 

 that any object is magnified. This will of course depend upon 

 a comparison of the real size of the object, with the apparent size 

 of the image ; but our estimate of the latter will depend upon 

 the distance at which we assume it to be seen, since, if it be 

 projected at different distances from the eye, it will present very 

 different dimensions. Opticians generally, however, have agreed 

 to consider ten inches as the standard of comparison ; and when, 

 therefore, an object is said to be magnified 100 diameters, it is 

 meant that its visual image, projected at 10 inches from the 

 eye (as when thrown down by the Camera Lucida, § 49) upon a 

 surface at that distance beneath, has 100 times the actual dimen- 

 sions of the object. The measurement of the magnifying power 

 of Simple or Compound Microscopes by this standard is attended 

 with no difficulty. All that is required is a stage-micrometer 

 accurately divided to a small fraction of an inch (the 1-lOOth 

 will answer very well for low powers, the 1-lOOOth for high), and 

 a common foot-rule divided to tenths of an inch. The micro- 

 meter being adjusted to the focus of the objective, the rule is 

 held parallel with it, at the distance of ten inches from the eye. 

 If the second eye be then opened, whilst the other is looking at 

 the object, the circle of light included within the field of view, 

 and the object itself, will be seen faintly projected upon the 

 rule ; and it will be very easy to mark upon the latter the appa- 

 rent distances of the divisions on the micrometer, and thence to 

 ascertain the magnifying power. Thus, supposing each of the 

 divisions of 1-lOOth of an inch to correspond with IJ inch upon 

 the rule, the linear magnifying power is 150 diameters; if it cor- 

 respond with half an inch, the magnifying power would be 50 

 diameters. If, again, each of the divisions of the 1-lOOOth inch 

 micrometer correspond to 6-lOths of an inch upon the rule, the 

 magnifying power is 600 diameters ; and if it correspond to If-^ 

 inch, 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 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 1-lOOOth-inch micrometer, 



' See Prof. Bailey's interesting memoirs in Vols. II and VII of the " Smithsonian Con- 

 tributions to Knowledge." 



