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V. — Note on the Besolution of AmpMpleura i^ellucida hy a ToIIes's 

 Immersion Ith. 



By Assistant-Surgeon J. J. Woodwaed, U. S. Army. 



In my paper on the use of the Nobert's plate, written in April 

 last, and published in the July number of this Journal, I found 

 myself compelled to make a few remarks on the objectives of Mr. 

 Tolles, of Boston. While comphmenting this maker on his excel- 

 lent workmanship, I felt constrained to say that I had not found 

 that his objectives excelled those of like powers by other first-class 

 makers. 



Late in June of the present year, however, I received from Mr. 

 Tolles a ^^th, the performance of which is so remarkable that I take 

 pleasure in drawing attention to it. 



This objective is so made as to work either dry or immersion, 

 and it is of its performance when used wet that I desire to speak. 

 Its magnifying power at 48 inches distance between micrometer and 

 screen (without an eye-piece) is 250 diameters when corrected for 

 immersion uncovered, 275 diameters when corrected for the thickest 

 cover through which it will work. It is therefore of rather higher 

 power than a ^th, but less than a ^th. 



Now, with this objective I find no difficulty in resolving Am- 

 ])lii])leura pellucida, the objective successfully displaying the trans- 

 verse striae on all but the most minute and difficult frustules. 



To illustrate the character of the performance, I send you here- 

 with two positives on glass from negatives taken by this Jth.* 



The first shows two frustules magnified 256 diameters. It is 

 of course necessary to use a lens, or a low power of the compound 

 microscope, to see the striae, which will be found to be quite sharp. 



The second shows the same two frustules magnified 920 dia- 

 meters. The striae can be seen with the naked eye, still better with 

 a lens. 



I send no paper prints of these negatives, because, on account 

 of the fineness of the striae, as seen with the above powers, they 

 would not be satisfactory. 



I add, however, a third positive, representing the same frustules 

 magnified 1140 diameters by the immersion xoth of Powell and 

 Lealand. 



This picture certainly shows that the new ath cannot be claimed 

 to supersede the highest powers at present in use, yet nevertheless 

 is not, in my opinion, injurious to the -Jth, for it must be mentioned 

 that the immersion y^th of Powell and Lealand, with which this 

 picture was taken, magnifies at 48 inches distance, without an eye- 



* The photographs are admirable. They are at the Society's rooms, where 

 they may be seen. — Ed. ' M. M. J.' 



