Monthly MicroscoplcalT 
Journal, March 1, 1869.J 
for the Microsco]^e, 
173 
in a peculiar way, so as to prevent injury from dust ; but when the 
highest powers then in use were set to the Hne " uncovered," their 
apertures became so far diminished by this correction, that the 
advantages in defining difficult tests were not so apparent as if a 
glass had been made specially for viewing them in this state, or an 
extra front lens had been adapted to existing objectives for this 
particular investigation. 
On the Observations requisite for Correcting Ohject-glasses. 
For this purpose, a particle of mercury is placed upon a slip of 
black glass. A piece of watch-spring, or the thin handle of a spatula, 
is held up at its end by the fore-finger of the left hand, and slapped 
smartly down on the mercury, which is thus beaten into powder, in 
the form of numerous minute globules. Of these, a larger size is 
selected for correction for colour, and a minute one for ascertaining 
the errors of figure and centering, and state of the oblique pencils. 
The globule must be illuminated by direct candle or lamp light, 
and not by daylight, as the latter will not allow perfect correction 
to be obtained. The hght requires to be set as close as it can be, 
and, of course, in the highest powers, where there is little distance 
in front. It must be very oblique ; but this is of no consequence, 
as it is not the globule itself, but the spot of light reflected from it, 
that is required to be seen. 
(To he continued,) . 
VOL. I. 
O 
