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On Microscopes and Microscopy. 69 



^The lenses of Powell lind Lealand did not, as I have before 

 ■said, meet my*anticipations; but since then I have met with bet- 

 ter, and as bearing upon another comparison of lenses, I will as 

 briefly as possible rotate what happened. Some time since while 

 in Philadelphia I had the good fortune to see some very recently 

 imported English lenses, in the possession of microscopists of 

 that city, who are too well versed in these matters to tolerate any 

 but the most superior glasses. Those of this character were from 

 Powell and Lealaiid. 



Another comparative trial being suggested, all the lenses were 

 adapted to a large fine English mounting, and it was thought 

 best to test them by direct light and with the podura-scale, the 

 greatest superiority resting of course upon the clearest and sharp- 

 est definition of the markings on that object. The y'^th inch 

 objectives were used. W 



It was evident at first that the angle of aperture of Spencer 

 was wider than that of the others, a larger field and more light 

 being of course the result. As to sharpness of definition the 

 opinions oscillated as at the other trial in Paris, first to one and 

 then to the other, as they always must when lenses of great su- 

 periority approach so 'near to each other. But the impression left 

 at last upon the mind seemed quite in favor of Spencer, as ex- 

 pressed afterwards by one who appeared equally capable and im- 

 partial in the matter. '' They are fully equal to th* best imported 

 objectives I have seen, and I will not say that they are not a little 

 better. I am inclined to believe that they are." 



It may seem strange to some, that the direct light was used in 

 this instance as a trial of the lenses. But it appears to me that 

 the merits of a lens are as well if not better exhibited by this 

 as by the indirect illumination. For in this case you have the 

 degree of perfection with which it transmits light, without so 

 much reference to the comparative angle of aperture, as to its ca- 

 pability of separating closely approximate rays emanating from 

 an exceedingly minute yet methodically marked object. Such 

 an object is a good scale from the Podura; and where several 

 microscopists are together and can see the same scale with the 

 different lenses, and can notice the difference of sharpness of 

 definition, the capabilities of the lenses can be very impartially 

 and correctly obtained. Some may urge that with any first 

 class lens, the whole structure of the scale can be made out ; this 

 IS very true, but this structure admits of being seen with differ- 

 ent degrees of distinctness. ^ 



Moreover when lenses approach each other pretty closely in 

 excellence, a difference may be perceived in distinctness of defi- 

 nition upon a. well known object when seen hydircf. light, 

 whereas that difference might well be disputed if those same 

 'enses were made to bear upon a surface, the markings of which 

 are with the utmost diffigulty, or not'a4»all, seen by indirect light. 



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