70 RemarJcs o)i a Tolles Itninersion y^ih. 



meut for uncovered o])jects, and find that it agrees closely with its 

 nominal power, it will show that the maker means what he says, 

 whether it happens to be |th or r^th. 



Mr. Wenham further says : From an early date Jths, ^ths, or 

 -,Vths, and some now approach yVths in power (also something 

 about steam-engines). Granted, but does all that prove anything 

 except the truth of my position ? Mr. Wenham has in this 

 endorsed my statement a second time, and as far as it being the 

 intention of the opticians to make yV^hs at the price of iths, and so 

 on, it is simply absurd. In my examinations of objectives of the 

 different makers, I find that Tolles and Zentniayer keep very close 

 to the standard, and I believe English opticians can do the same 

 if they choos^. 



I wish to ask Mr. Wenham if he does not believe all good 

 opticians can put their objectives right on the mark ; that is, if they 

 will; and when that is done let qualitij decide the question of 

 superiority ? 



Museum of Comparative Zoology, 

 Cambridge, Mass., Dec. 26, 1871. 



VII. — BemarJcs on a Tolles Immersioii -rsth. 

 By Edwin Bicknell, 



I NOTICE in the ' Monthly Microscopical Journal,' vol. vi., page 

 290, a Note on Tolles' Immersion ^th communicated by Mr. Slack, 

 from a letter written by Dr. Woodward to Mr. Slack ; the con- 

 cluding paragraph of which is as follows : — " I wish I could speak 

 as favourably of Mr. Tolles' higher powers. They are very good 

 indeed, but I have yet to see one of them which will rival the so- 

 called reth immersion of Powell and Lealand." I have at present 

 by me a Tolles' ygth which was formerly a dry Ygth, and was " con- 

 verted " into immersion in August, 1868 : its power when the com- 

 binations are closed is yVth, ang. ap, 170^ ; when the combinations 

 are opened for uncovered objects its power is under a y^ih and its 

 ang. ap. 155'^ ; although somewhat under, I prefer to call it a rVth. 

 I have this morning, December 25, 187 J, by means of sunlight 

 rendered monochromatic by the copper solution, and an achromatic 

 condenser of 145^, stopped off all but the extreme obhque pencil, 

 resolved, counted, and measured the three last and most difiicult 

 diatoms on Moller's " Probe Platte," viz. Navicula crassinervis, 

 Nitschia curvula, and Ampliipleura ])ellucida : these are all in 

 balsam. I have a portion of the slide oi AmfhiiHeura i)€llucida, 

 which (I believe) Dr. Woodward first jihotographed (whicdi was 



