254 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
nut, so that it can be used without having any influence on the 
fine-adjustment. To bring the fine-reading apparatus into play it is 
only necessary to turn one screw, which then by means of a screw seg- 
ment presses on another screw. With this instrument it is possible to 
measure the depth of the furrow produced on a piece of glass by 
making a finger-stroke upon it with some fine polishing powder. 
For the cutting of normal screws the author recommends a tool 
which he calls a “ Schneideneisen.” The teeth of the tool cut the thread 
quite gradually up to the right depth, for the first tooth only cuts a 
slight dent in the material, which each of the succeeding teeth deepens 
by a certain amount. 
Construction of Silvered Lens Mirrors. — Mr. Edward M. Nelson 
contributed the following to the meeting last December : — 
Although the idea of making a mirror by silvering one side of a 
convex lens is an old one (probably dating back more than 100 years),* 
it has more than once been reinvented and brought forward as something 
quite new. It is, however, strange that such a simple, excellent, and 
cheap contrivance, and one so eminently suited for microscopical pur- 
poses, should have received so little attention. 
With the object of keeping the price of the Farmer’s Microscope as 
low as possible, I had two silvered equiconvex lens mirrors fitted to it, 
one being for substage, and the other for superstage illumination. 
While experimenting with these mirrors, their practical usefulness 
so greatly impressed me that I at once proceeded to investigate the 
theory of their action. On looking up the subject, I was fortunate to 
find in an article by Sir D. Brewster a passing notice of a reflecting 
telescope, designed by the late Astronomer Boyal, which was composed 
of silvered lenses. 
Referring to the original paper in the 1 Cambridge Philosophical 
Transactions for 1822,’ we find that Sir G. Airy was led to investigate 
the optical principles of lens mirrors, both on account of the difficulty 
he experienced in obtaining flint discs for achromatics, and also because 
he found that metallic specula were liable to tarnish. 
He had two Cassegranian telescopes made from formulse he had 
computed. After trial he said that “ the image of a star or planet was 
surrounded with radiations which made the telescope quite useless for 
practical purposes ; ” the reason of this he did not discover, but he did 
not think it arose from any residual chromatic error. 
In his paper the foci, radii, and refractive indices of the glasses are 
not given, but there is an able mathematical analysis of the destruction 
of the aberrations of one lens mirror by those of another, which was of 
assistance. 
The problem now before us, viz. the aplanatism of a single-lens 
mirror for parallel rays independently of chromatic aberration, though 
fortunately simpler, nevertheless requires a cubic equation for its 
mathematical solution. I say mathematical solution, because with 
much additional labour, a result sufficiently near for all practical pur- 
poses might be obtained without mathematical knowledge. The data, 
See this Journal, 1890, p. 88. 
