96 CORRESPONDENCE. 



mersion," tliat it reads like a precise inversion of the real facts. 

 I will not enter at present on the superb deiining and resolving 

 power of this glass (which remains clear and crisj) under E ocular), 

 but confine myself to the question of penetration and facility of 

 working. 



Having lately, through the kindness of Mr. Curties, had a selec- 

 tion for examination of Zeiss' J-ths and yj ths, I took the opportunity 

 to make an exhaustive comjiarison of the relative penetration of these 

 low-angled glasses (only 105°) against the Powell and Lealand -gth, 

 fully expecting to find the latter surpassed in this respect. I era- 

 ployed every kind of object which an ith could focus. I first took 

 the coarser and more uneven diatoms : then spores, pollens, blood- 

 disks, and the like ; followed by the finer sections of vegetal and 

 animal tissues, down to uncovered sections of rock containing crystals 

 or arborescences in different planes. In every case, even these last 

 extreme ones, the new -|th gave at least as fine a general or per- 

 spective " picture," while the definition was of course far sharper. 

 Hence I feel confident that every class of workers, histologists as 

 well as diatomists, may derive the full benefit of this imin-ovement in 

 the objective, which seems to me the most important advance since 

 the days of Andrew Koss. It is true that the working distance of 

 this glass is small for an " immersion," being about the same as 

 ordinary dry ^ ths of large angle. 



I may add that some time ago Messrs. Powell and Lealand made 

 me a ^-incli of 40° for binocular use, which I find invahiable from 

 its penetration and beauty of definition ; also a fine 1-inch of 20°. 

 I name this because these glasses are not in their published list, and I 

 should have availed myself of them long before had I known they 

 were procurable. 



Having referred to the Zeiss lenses, I ought in justice to add that 

 I find them (the ^zft^i more especially) well worthy the attention of 

 all who may not require the highest attainable perfection. The cor- 

 rections are well made, and the field flat, with plenty of light. The 

 magnifying power of the Zeiss |th and Powell and Lealand immer- 

 sion Ith is the same, and measured at 10 inches with camera lucida 

 on Eoss's B ocular is x 640 ; that of the Zeiss ^V^^j X ^^^ '■> *^^ 

 adjusting collar being set half-way in the " run " in each case. 



As a chemist, I would emphatically urge all who use immersion 

 glasses to employ distilled water with them. Mere exposure to air, 

 especially when aided by gentle warmth, causes ordinary waters to 

 deposit an insoluble film of carbonate of lime, and when evaporation 

 takes place during prolonged work, sulphate of lime may follow 

 suit, and this cannot be dissolved ofl" even by acid. It is obvious 

 that any such coating on the front lens must sensibly impair its 

 efficiency. 



I am yours obediently, 



Frederick W. Griffin, Ph.D. 



