60 On Microscopes and Microscopy. 



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 But there is another reason which I wish to nree in favof'of 



direct light being used in comparativ^e trials, where instriimetits 

 are together. Every one is aware of the factj that in practising 

 with his own instrument at diiferent times, upon test-objects with 

 oblique light, there are at times when with a certain obliquityj 

 the test appears most excellently well brought out, while there 

 are other times when apparently with the same amount of light 

 and obliquity, the test is seen scarcely at all. Now all this arises 

 from a slight yet real difference of the manner in which the ob- 

 ject is illuminated, which, although otherwise imperceptible, 

 causes that deficiency of a perfect shadow so essential to a clear 

 definition. Now in a trial of instruments at a distance apart, 

 this mode cannot be pursued, because a difference of distinctness 

 must be seen and cannot be expressed ; but I am the more per- 

 suaded that the comparative superiority of instruments, widely 

 separated and upon different mountings and with dissimilar modes 



be 



I have 



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seen lenses when tried upon a certain mounting cast aside as in- 

 ferior to others upon a different mounting, whereas, subsequently 

 when they both were put upon a single mounting befitting each, 

 the former were found the superior. 



I have thus informed you of the results of the comparative 

 trials of Spencer's lenses at which I have been present; and 

 when we regard the untutored labors of Mr. Spencer as thus 

 coming so successfully into competition with those of the long 

 experience of the first artists of Europe, we may well look upon 

 our countryman with pride. 



Before, I close I will allude to the general character of micro- 

 scopes of different makers, as it has appeared to me during the 

 favorable opportunities presented at these different trials. 



As a whole, the English instruments are very fine, and from 

 no other makers can be obtained such exquisitely wrought brass 

 mounting, united to quite excellent lenses. But they certainly 

 have the objection of want of simplicity which makes them un- 

 wieldy, and almost fixtures wherever they may be placed, and 

 attaches to them a price that places them beyond the reach of 

 many who would use them best. 



Moreover I cannot think their mountings so well suited for 

 real work as some others; but they are most excellently adapted 

 for that class of microscopists who are not desirous of being work- 

 er^ so much as of being amateurs in this department of ^ience. 

 j|And although there are many times when all the nice mechan- 

 ical arrangements of an English mounting may be almost indis- 

 pensable, yet the man who does the real work, and from the la- 

 bors of whom this science is to progress, will not use them once 

 in a hundred times. 



But if complexity is the excelling point with the mountings of 



PPO 



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