176 REPORTS FROM THE SECTIONS. 
The origin of the correction for varying thicknesses of glass 
covers must be credited to the late Andrew Ross, who showed the 
imperative necessity of compensating the error arising from the 
difference between the front of the lens and the thin cover when- 
ever high powers were used. Now if we can suppose a fluid 
having the same refractive and dispersive indices as the glass of 
which the front lens of the objective and cover is composed, the 
end in view will be attained. 
A suitable medium for the purpose was by no means readily 
nd ; it was not until no less than sixty-three different oils and 
thirty other fluids had been tested that it was ascertained that oil 
of cedar-wood, though not absolutely identical with crown glass, 
ose, g 
oblique light. With central illumination, however, it was im- 
proved by the addition of one-fourth of oil of fennel seed. 
The new oil immersion objective was first made of in. focus, 
Zeiss has since produced a in. on the same principle, but I have 
not heard of the latter being superior in any way to the former. 
As far as externals go there is nothing in the appearance of 
the new objective to especially recommend it; the brass-work is 
The angle is extreme—113° balsam, or larger in the proportion 
ir 
and also indicates the highest resolving power hitherto attained. 
It has a very large working distance, that between the front lens 
_ And now as to its performance. Undoubtedly the first adv : 
tage that commends itself is the absence of any necessity for 
cover ijustment—you ; 
arrange your light, and the best result is at once before you ; we 
- alas! with the departure of one trouble comes the advent 
eae heer T- if ae 
gs) ST aR a Re ae cgnn T e 
eit 
Nie eee 
