OIL-IMMERSION OBJECTIVES. 131 
tion. In conclusion, he said that in the present state of science it 
would be well if, instead of studying a whole host of phenomena, each 
took up a particular branch, and made it a special study, for by so 
doing they might throw some light upon that which at the present 
time was obscure. 
OIL-IMMERSION OBJECTIVES. 
NY one who has once used the homogeneous immersion 
objectives, not alone for the resolution of diatoms, but for the 
investigation of minute pond life, will, we imagine, agree with us that 
the superiority of their performance over dry objectives is such as to 
give an impetus to their increased use. When first introduced the 
price was excessively high—only a favoured few having them 
within their reach, but thanks to Herr Leitz, of Wetzlar, we can now 
purchase a +), for £5, and a ', for £09. 
These objectives perform very satisfactorily upon most objects, 
and those which have passed through our hands in the verification 
department have been very fair glasses. 
Of course we do not mean to say that they come up to, in every 
particular, the high-priced lenses of Powell and Lealand, but if we 
had to institute a comparison we should say that to the lenses of this 
celebrated firm they stand in the same grade as the second series 
of some makers do to their first series. 
The numerical aperture of these lenses is what may be termed 
moderate for oil immersions, and there is plenty of working dis- 
tance ; the ;4, has a working distance sufficient to run over all the 
mounted objects in our cabinet; it has an aperture of 1°26 or 
112° balsam angle. In magnifying power it is more nearly a 35> 
than =, and the collars of several of the monads were shown 
beautifully with it. 
The 1; we examined, singularly, had less working distance than 
the ;4,, though it possessed exactly the same aperture, and this 
glass would not work over our thickest covers. The 34; was, how- 
ever, much better corrected than the ;, and consequently would 
stand deeper eye-pieces. With the 74, we have had much expe- 
rience, as several of them have passed through the verification 
department, and we feel bound to say that the purchase of this 
glass is a good investment. 
The 54; has a magnifying power equal to a real ,';, and has 
the working distance of a Zeiss dry ,};; its aperture is 1°22 or 
107° balsam angle. We have lately had an opportunity of 
