426 SUMMARY OF CURRENT RESEARCHES RELATING TO 
scale the beads rather tend to form single rows. In many places the 
beads seem to be oval, but this I bold only to show that the beads are 
not properly separated. In the photograph the change from the oval to 
the double bead is easily traced, but whether this is optical or physical 
I am unable to say positively.” 
Mr. Yereker writes to say, under date March 15th, 1892, “I regret 
that I was away in the north of England when my paper on the Podura- 
scale was read, but if you will allow me I should like to answer one or 
two points in the discussion which followed. 
I used a water-immersion of Nachet’s, No. 9, which is nominally a 
1/14, hut really I think a 1/13, for the resolution with the immersion 
paraboloid, as owing to its great extent of correction I was able to work 
on a dry slide; and also because my 1/12 oil-immersion of Reicherts 
would not work satisfactorily on a dry slide. I may say, however, for 
those who wish to use an oil-immersion, that Powell and Lealand’s new 
apochromatic oil-immersion will work on a dry slide, and I inclose you 
a photograph of Podura plumbea on dark ground done with it. Un the 
other hand I am doubtful whether there is much gain in so using an oil- 
immersion. As for isolated featherlets, I have just got a photograph with 
some, and with all due deference to others, there are featherlets missing 
on the scales on the slide ; but I think, owing to the thickness of the pile 
in some places, it would be very hard to find out where a solitary 
featherlet was missing. 
In reference to the beaded structure, it was photographed with direct 
light, and on the focusing screen the beads with Reichert’s oil 1/12 stand 
out very clearly against the rest of the scale ; in the slide I used they 
were quite distinct from the featherlets, which indeed are not in focus at 
the same time as the beaded structure. The scale used was a specially 
selected one kindly lent by Dr. Edmunds and by a well-known preparer.” 
A simple Apparatus for Photomicrography.* — Mr. T. H. Muras 
describes an apparatus for use on an ordinary camera : — “ Its chief advan- 
tage over the use of a Microscope with the camera being that the ordinary 
use of the Microscope is not interfered 
with, so that the objects can be ex- 
amined in the latter, and selected 
parts immediately photog'aplied. A 
brass disc or adapter A, shown in 
section in the illustration, screws into 
a flange on the camera, and has a 
screw-threaded aperture to receive 
ordinary object-glasses. The disc 
carries a triangular rod B, about 3^ in. 
in length, on which a square stage C 
slides, or can be clamped in any posi- 
tion by a screw D. The stage has a 
central opening for light and springs for holding a slide. The back of 
the camei a is movable to a distance of 20 in. by a screw ; but an ordinary 
quarter-plate camera, with a rack and pinion, serves almost as well, the 
magnification then obtainable being less, owing to the shortness of 
Fig. 55. 
Engl. Mechanic, lv. (1892) p. 61. 
