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On Microscopes and Microscopy. 63 



iThen again, such an abundance of light removes much obscu- 



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nty from, gnd renders the use of, very high powers, quite available. 

 ^And the power of 1500 diameters with Spencer's lenses is quite 

 as clear and distinct as one-third that number with the French 

 and many English. Every one knows how much belter it is to 

 be able to control the amount of light by a diaphragm, than to 

 have the lenses control it for you. These are the grand merits 

 of Mr. Spencer's lenses, which are the better shown off by the 

 very excellent manuer in\vhich they (the lenses) are mounted. 

 But with these manifold advantages, there are some disadvan- 

 tages which have quite naturally resulted froni the mode of lens- 

 working pursued by Mr, Spencer. The lens being wrought with 

 an angle of aperture, at the expense of its aplanatism, the focus 

 IS made, in the high powers impractically short, so that although, 

 for instance, one may be using or wish to use the ^\i\i ob- 

 jective of 140^ or 150° angle of aperture, with the advantages of 

 much light, &c., he will find that scarce any but objects specially 

 prepared with the thinnest of covering glass, can be placed be- 

 neath it. With one who wishes to work with his lenses this is 

 a serious difficulty, and I have thought, that unless this can be 

 remedied, the possession of high objectives having more than 

 loO^ angle of aperture is not desirable for working microscopists. 

 The attention of Mr, Spencer has been directed of late to this 

 point, and it would appear that after some experimenting, he 

 expects to go even beyond the solution of this difficulty. In 

 a recent letter, answering some inquiries on this subject, he 

 makes the following remarks: — "The remark you made that 

 an increase of angle of aperture tends to shorten the working 

 focus, is true of the present formulas. I do not remember that I 

 mentioned this subject in a former letter, at any rate, my atten- 

 tion has been turned to it, and sometime since I began an ob- 

 jective under a new formula, the express design of which is to 

 obviate this defect and to secure a working focus — for even a 

 i%}h or ^'^th inch objective, equal to the common yVths. J think 

 / this can be done and retain the maximum angle of aperture. It 



Goes not follow that an increase angle of aperture interferes w^ith 

 ^J^ corrections of figure and color to such an extent, as to set the 

 skill of the artist at defiance. It doubtless increases the difficulty 

 ^^ 'investigation^ and to some extent the execiition.'^^ 



,^uch news is more than welcome to all lovers of microscopic 

 science — and considering what has already been accomplished, 

 ^e can look forward with both hope and pleasure to the time, 

 Jiot far distant, when complete success shall have crowned trie 

 tabors of our countryman. 



I rusting that the interest of the subject will be something of 

 an apology for the length of this letter, I am yours truly, 



Boston, May, 1851. 



W. L Burnett. 





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