MANAGEMENT OF THE MICROSCOPE. 173. 



a sufficiently high amplification, is seen to be covered with the peculiar 

 markings shown in Plate n., Figs. 2, 3, which are sometimes designated 

 ' spines,' but are more commonly known as ' notes of admiration ' or 'ex- 

 clamation-markings.' These should be clearly separated from each other, 

 and their margins well defined. An Objective of small angle (such as a 

 l-4th inch of 60) will show the ' spines' dark throughout; a l-4th inch 

 of 100 will show a light streak extending from the large end, down the 

 centre of each marking; and a further enlargement of the aperture will- 

 show an extension of this streak through the entire length of each 

 ' spine.' The degree in which these markings retain their brightness and 

 distinctness under deep Eye-piecing, may be considered a most valuable 

 test of the excellence of the defining power of the Objective. As it is 

 impossible that large-angled Objectives used 'dry/ should be perfectly 

 corrected for spherical aberration (so as to possess the greatest possible 

 defining power) without some residuum of chromatic aberration, all the 

 best defining glasses will show the thick part of the spines tinged with 

 either blue or red. Perfect Achromatism, on the other hand, is only 

 attainable with 'dry' lenses at some sacrifice of resolving and defining 

 power; and many Microscopists prefer to keep the latter to their highest 

 point, even at the expense of complete color-correction. Most Physiolo- 

 gists, hower, will prefer the highest attainable achromatism, at some sac- 

 rifice of aperture. But it .is one of the advantages of the ' immersion- 

 system,' that the residual aberrations of even large-angled Objectives can 

 be much more perfectly compensated than they can be in ' dry ' Objec- 

 tives; so that on this as on several other accounts, their use is to be re- 

 commended whenever permitted by the nature of the research. 



163. 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 ten inches from the Eye (as when 

 thrown down by the Camera Lucida, 94, upon a surface at that distance 

 beneath), has 100 times the actual dimensions of the object. The mea- 

 surement 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- 100th will answer very well for low powers, the l-1000th for high), and 

 a common foot-rule divided to tenths of an inch. The Micrometer 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 through the Microscope, the circle of 

 light included within the field of view crossed by the lines of the Micro- 

 meter will be seen faintly projected upon the rule; and it will be very 

 easy to mark upon the latter the apparent distances of the divisions 011 

 the Micrometer, and thence to ascertain the magnifying power. Thus, 

 supposing each of the divisions of l-100th of an inch to correspond with 

 1 J inch upon the rule, the linear magnifying power is 150 diameters': if 

 it correspond with half an inch, the magnifying power is 50 diameters.. 



