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if at all practicable, therefore we must seek for the nearest approxi- 

 mation to it. Single lenses of any kind form an image of the source 

 of light, which, being coincident with the object under observa- 

 tion, mingles with it, and tends to confusion. I tried the image 

 of a white disc of plaster of Paris, of a plate of unpolished silver, 

 and other substances ; but after a variety of trials, I ultimately found 

 that the pencil emergent from the eye-piece of a telescope, when ad- 

 justed for distinct vision of a distant object, was the closest ap- 

 proximation I could obtain to that for which I sought ; and accord- 

 ingly I have adopted it with, as I conceive, very great advantage. 

 My present arrangement consists of a disc of grey glass, strongly 

 illuminated, an object-glass (so, for convenience, I call the lens 

 nearest to the grey glass) and an eye-glass, that from which the 

 illuminating beam passes to the object. 



" These are each adjustable for distance, and should be so ad- 

 justed that, looking through the eye-glass you get a distinct image 

 of the grey glass. The illuminator, thus arranged, is placed behind 

 the object to be examined, this latter must then be adjusted for dis- 

 tinct vision through the microscope; a low power, say one inch, is 

 convenient for this purpose ; next, the illuminator must be so ad- 

 justed, as to distance behind the stage, that the circular spot of light 

 which would be used, if a dynameter was applied to measure the 

 power of the miniature telescope, shall be perfectly coincident with 

 the object, which is then ready for examination with any power you 

 please to apply. 



" Such is the state in which I now use this illuminator, and I 

 think I may say that all who have seen its performance, amongst 

 whom our respected President has had the most frequent opportu- 

 nities, will agree with me as to its superior efficiency. 



" It is now some ten or twelve years since I first tried this ar- 

 rangement ; for a very long interval I was obliged to discontinue 

 the use of the microscope, and it is only within a few months I have 

 again returned to it. This much I feel it necessary to say in ex- 

 planation of not having before this given any public description of 

 the arrangement, and still more for not having further developed 

 and improved it, in which respect much still remains to be 

 done, — chiefly in determining the ratios of the lenses composing 

 the instrument, in relation to the diameter of the illuminating 



