234 Aperture of Object-glasses. 
Professor Govin has proposed to associate the measurement of 
the angle of aperture with the simultaneous view of an object 
distinctly defined, like the flames of two candles placed asunder, or 
two white strips separated on a black screen, to the limit of distinct 
visibility ; the angle from these two points to the focus of the 
object-glass will represent the aperture. The microscope is thus 
converted into a kind of telescope, by means of a pair of lenses 
over the eye-piece similar to what is known as Ross’s “ examining 
glass.” Unfortunately for the success of this plan, different optical 
combinations at the eye-piece give different results by elongating 
or shortening the conjugate focus, otherwise the measurements 
would be very satisfactory. I am in hopes of finding a remedy for 
this. Proposals have also been made for testing aperture by 
placing a lamp-flame at the back of the object-glass, which remains 
a fixture, and rotating a compound microscope round the coincident 
foci as a centre, and measuring the arc traversed. This is exceed- 
ingly inaccurate, as the aperture of the rotating glass is superadded 
to the other. I am, however, constantly in the habit of using this 
plan for testing the perfection of the image formed by the oblique 
pencils, in order to ascertain whether they are good enough to remain 
or should be stopped. When the aperture of the object-glass is very 
large, and the marginal pencils free from error, a singular optical 
illusion is produced. On looking towards the front of the object-glass 
under test, in a line with its surface, a distinct image of the flame 
will appear to stand out suspended in air ; this appearance is caused 
by the degree of aperture or the oblique pencils of the examining 
microscope forming an image where none should apparently be 
seen. I offer these remarks as a caution that whether apertures 
are tested either in air or water, errors are liable to creep in. At 
present I have no remedy to suggest, as the investigation remains 
to be completed. 
Since writing the above, I have to announce that Mr. Tolies’ 
object-glass has been placed in the hands of Dr. Lawson. I shall 
not, however, test the aperture in either water or balsam, till the 
return of Professor Markoe from the Continent, in order that he 
may be present, by which time everything will be in readiness, as I 
perceive that a special contrivance will have to be adapted for the 
aperture in balsam, to avoid drowning the glass, or prevent it 
from getting into the fittings, as I should not like to take the risk 
and trouble of cleaning it out again. This may be avoided by 
confining the balsam to a small spot on the front, leaving the sides 
intact. 
