172 Construction of Ohject-Glasses [SrnlT.MSiS 
the cut. The correcting thickness should be thrown in the first 
lens. If they are set in contact, the magnifying power will be 
nearly as their sum; they may 
therefore be made of double the 
radius, and consequently nearly 
twice the diameter, which, of 
course, would lessen the practical 
difficulty of working a -5^ ^h, and 
enable us to go even beyond this 
power. A partial experiment with 
a Jth, having this " doublet " front, 
has proved that perfect correction 
for colour is the result. But 
in the form tried, the spherical 
aberration was so considerable, as 
to require an entire reconstruc- 
tion, for which I have now no 
leisure ; and though the entire 
success of the idea is yet unproved, I venture to record it, in case 
I may never be able to take up this subject again, as I am of opinion 
that a very perfect object-glass may be made of this form. 
Eecently some excellent glasses have been made, as the so-termed 
" immersion lenses." These combinations are under-corrected, and 
not suitable for use in any other way. The plan is an old one, and 
objectives were constructed on this principle by Amici and Koss 
many years ago. That such lenses give brighter and clearer defi- 
nition, with the highest powers, from the -pVth upwards, is unques- 
tionable ; but the efiect of the water and covering-glass is precisely 
the same in its corrective action as additional thickness thrown on to 
the front lens. The interposition of water doubtless tends to neu- 
tralize errors of surface and polish, and more light is transmitted in 
consequence; but, as organic preparations are generally not so 
minute in their structure as to require the habitual employment of 
such object-glasses (which are useless without the water film), it will 
probably be found, that the inconvenience attached to their use will 
not compensate for their advantages. Allowing that they perform 
better on the Diatoms and other tests, it would be preferable to 
mount these objects, in which special discovery is required, without 
any covering-glass, and have an objective of ^^'^th constructed with 
a thick front, specially corrected for uncovered objects only. This 
having no adjustment, the best definition would always be a matter 
of certainty, which cannot be the case when the covering-glass, with 
its varying thickness and errors, forms part of the optical combi- 
nation. 
The late Eichard Beck strongly advocated this mode of viewing 
Diatoms, and had a series of them mounted as uncovered objects, 
