518 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Lemardeley’s Photographic Microscope.* — This apparatus consists 
simply of an objective, a condenser, and stage for holding the prepara- 
tion, all attached [to a disc which is mounted directly upon the 
camera. It is thus a Microscope minus the illuminating mirror and 
tube carrying the eye-piece. The clips on the stage for holding the 
preparation are below instead of above, so that the preparation may be 
always placed in the same plane whatever the thickness of the cover- 
glass. The apparatus is provided with a system of automatic focusing 
for the use of those who are not familiar with the Microscope. 
(5) Microscopical Optics and Manipulation. 
A Suggested Method of Increasing the N.A. of Old Achromatic 
Object-glasses.^ — Dr. Piffard, in a paper in the ‘ Medical Record ’ for 
March 24, calls attention to what he claims to be a “ simple means of 
increasing the aperture and improving the performance of some immer- 
sion-lenses.” He says, “ About seventeen or eighteen years ago I pur- 
chased of Powell and Lealand a 1/4-in. water-immersion, rated by its 
makers as having an angular aperture of 143°. This would correspond 
to a numerical aperture of 1*26, and is, I believe, the highest aperture 
ever given to a water-immersion lens. Two or three years ago I dis- 
covered that it would work perfectly well with cedar-oil, which raised its 
aperture to N.A. 1 * 44. 
More recently I have ascertained that, by closing the systems as far 
as the correction -collar would permit, and shortening the tube-length to 
about 155 mm., the objective would correct perfectly with monobromide 
of naphthalin. Not only was the over-correction introduced by the 
monobromide completely neutralized, but the character of the image 
remarkably improved. 
I next investigated the possibility of error in the makers’ statement 
as to the angular aperture, and found that with systems open and water 
contact the angle measured 143° ; with systems half closed and oil 
contact the angle was 144 ° ; and with systems entirely closed, using 
monobromide contact, the angle was also 144°. Applying the usual 
formula N.A. = n sin u the result gave N.A. = 1 * 56, the highest 
aperture, so far as known to me, that has ever been obtained with an 
achromatic lens, working with crown-glass slides and covers. With 
the lens used in this manner I have resolved the Arrijphipleura pellucida 
on a dry mount, and this has been separately verified by my friend, 
Mr. H. C. Bennett, of this city. I mention this fact in order to show 
that the lens remained in a state of perfect correction, and stood up to 
the high eye-piecing which was needed to amplify the image to a point 
that would enable the resolution to be seen. 
A duplicate of my lens and a Powell and Lealand 1/8-in. water- 
immersion N.A. 1*26, the property of Dr. William Stratford, were also 
found to work correct perfectly with the monobromide. A 1/6-in. oil- 
immersion N.A. 1*50 of the same makers failed to do so, possibly owing 
to insufficient range of adjustment. 
Two American oil-immersion lenses of 1/10-in. equivalent focus, 
* English Mechanic, lix. (1894) pp. 383-4. 
t Medical Eecord, xlv. (1894) pp. 362-4. 
