242 CORRESPONDENCE. 



method still carried on in the workshops for effecting new com- 

 binations may be dispensed with, by a previous computation " not 

 difficult." 



Of course there is but little use in such a calculation if anticipated 

 by the practical result to which it is applied. 



Yours very truly, 



F. H. Wenham. 



Zeiss' Objectives. 



To the Editor of the ' Monthly Microscopical Journuh^ 



66, KiNGSDOWN Parade, Bristol, April 10, 1876. 



Sib, — If in the note published in your February number (p. 96) I 

 had invidiously pitted the Zeiss dry objectives against Powell and 

 Lealand's new immersion, I must have acknowledged myself open to 

 Mr. Hickie's courteously expressed strictures. As, however, my remarks 

 can hardly be so interpreted, I demur to the indictment. I went, in- 

 deed, a little out of the way to do justice to the Zeiss lenses (as I 

 considered they fully deserved), for my concern was to vindicate the 

 new ^th from an unmerited charge of want of penetration, which is 

 practically tantamount to limited usefulness. Now penetrative power 

 is usually considered to increase as the angular aperture is lessened ; 

 a belief which these Zeiss glasses of remarkably small angle seemed 

 capable of putting to a severe test. It was in the one quality of 

 penetration that I compared the |th with the Zeiss lenses, and these 

 were bound to be dry ones, as no low-angled immersions are made 

 either by him or others. That the Powell and Lealand glass should 

 fully equal them in penetrative power was, I confess, an agreeable sur- 

 prise, which I think few would have anticipated. I made no com- 

 parison between the relative resolving powers, as this would have 

 been manifestly unfair towards glasses of little more than half the 

 angular aperture of the new |th ; but I may say that even in tliis the 

 dry Zeiss lenses appear to be satisfactory. I have not tested them 

 very critically myself, but while I was at Sidmouth last aatumn, the 

 Eev. Lord S. G. Osborne showed me fine " resolutions " with a dry 

 Zeiss xV^li skilfully manipulated, and he writes me that he has since 

 greatly surpassed them by the aid of some newly-devised arrange- 

 ments. 



I was led to mention the amplification of the Zeiss glasses, not only 

 because, as Mr. Hickie indicates, the foreign numbers convey no in- 

 formation to us, being taken at a much shorter distance than our stan- 

 dard 10 inches and with lower eye-pieces, but also because my figures 

 do not agree with those given by Zeiss. He states the diameters 

 yielded by his ^th and -j^^th to be x 330 and x 500, whereas I 

 found them as X 330 : X 451, showing that one must have been con- 

 siderably over- or the other as much under-estimated. Possibly 

 different specimens of his lenses, nominally the same, may vary in 



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