The Eye and the Microscope. 433 



quite wrong, and the common exclamation, " That must be a 

 very powerful microscope,'''' is a frequent indication of this mis- 

 take. High powers are needed to exhibit details that can have no 

 meaning, and often no beauty, for those whose minds have not 

 been previously prepared to appreciate them by a broader 

 acquaintance with the complete structure of which they form a 

 part. Thus the same steps which the microscopist should take 

 for the preservation of his eyes, lay the best foundation for the 

 more difficult operations, in which the employment of higher 

 objectives and of niceties of illumination become indispensable. 



When a good set of experiments have been made with an 

 inch or two-thirds objective, and the difficulties of microscopic 

 vision have been sufficiently conquered, a similar course should be 

 commenced with a quarter or a fifth object-glass. Objects that 

 are severe tests of the perfection of such glasses should be avoided 

 until those which are easier have been successfully displayed. 

 The scales of the Vanessa urticoe, and that pretty diatom the Pleu- 

 rosigma hippocampus^ will do admirably for a beginning, taking 

 them in the order mentioned. The butterfly scales require a 

 clear, steady light sent through with little obliquity, and the 

 mirror must be turned so that the illumination leaves their edges 

 clear all round. If one side appears shadowy, there is some 

 fault in the angle at which the light falls ; and if the unoccupied 

 part of the field is too brilliant, either the light must be reduced 

 by turning the lamp a little lower, or the distance of the mirror 

 from the stage must be altered, or the spermaceti screen inter- 

 posed. 



The Pleurosigma hippocampus will give good practice in 

 regulating the obliquity of the rays from the mirror. It is a 

 very easy object for a good manipulator, but a beginner must 

 not be surprised if he makes a hundred trials before he can 

 quickly and readily show the two sets of lines as plainly as he 

 can see the pattern of an engine-turned watch. As a rule, no 

 more light should be used in displaying any object than would 

 be sufficient to imitate the brightness of a larger object of the 

 same kind as seen in the diffused illumination of a fine day, and 

 it is often desirable not to exceed that of a shady room in the 

 summer time. When the lamp and optical contrivances have 

 been judiciously arranged, an increase or decrease of light can 

 be obtained by turning the wick a little higher or lower, and if 

 an observation is prolonged, less light will suffice than for a 

 temporary glance, as the eye is more willing to accommodate 

 itself to a moderate illumination than to grow accustomed to 

 anything like a glare. 



A magnification of two hundred linear or upwards can only 

 afford easy vision with objects that are very flat, as slight inequa- 

 lities of surface inevitably throw the salient, or retreating, por- 



