(oe 
TRANSACTIONS OF THE SECTIONS. 9 
to give to the reflecting telescope every possible accession of improvement which the 
progress of art or science placed from time to time at our disposal. The two points 
of admitted inferiority of the reflector being, a greater liability to tarnish than glass, 
and less intrinsic brilliancy of the reflected pencil of light; the author had suc- 
ceeded (so far as the sma!l speculum of the reflecting telescope is concerned) in 
entirely obviating the former objection, and in very much lessening the other. Regard- 
less of the failure of an attempt, made years since, to construct a reflecting telescope 
of glass surfaces quicksilvered, he concluded, from his own experience, that such 
surfaces could be made equally perfect with those of speculum metal; while by 
silyering (not quicksilvering) that surface required to reflect, a great increase of light 
would result, thus producing for the small reflector of the telescope a mirror as 
imperishable as glass, and, in reflecting power, approaching the transmitting power 
of alens. The author explained why, instead of using this reflector in its simplest 
form, viz. that of a lens of equal thickness silvered on one side, he preferred an 
achromatized compound of two lenses, cemented and silvered, and exhibited such a 
compound, which, he stated, had on trial performed perfectly. He next proceeded 
to describe in detail his proposed application of the same principles to both small 
and large specula of telescopes (where such were of moderate dimensions), as also 
an improved form of the prism of total reflexion applicable to Newtonian telescopes 
‘of the largest dimensions. This latter is a prism of divergent or concave power made 
aplanatic, or at least achromatized, reducing the convergence of the rays coming 
from the large speculum, and also the size required for the prism in the same arbi- 
trary proportion (two or three times being suggested) ; the required magnifying 
power being obtained by a proportionally lower eye-piece. The author next pro- 
ceeded to discuss the respective merits of the several varieties of equatoreal mounting 
as applicable to large telescopes. The first variety, or long polar axis (biforked or 
not), he rejects from its necessarily great length and consequent unsteadiness. The 
second, or large-cone polar axis, supporting the telescope in a bifurcation prolonged 
beyond the upper bearing, he would also reject, from the enormous weight of such 
in proportion to the telescope carried,—4 tons being stated as the moving mass 
in the case of a telescope of only 8 inches diameter. The third, or German 
variety of construction, the author considers, in its general type, as preferable to all 
others; and he has therefore devoted much attention to its improvement. By a 
system of internal counterpoise, he has reduced the direction of the pressure of the 
declination axis (with its appendages, including the telescope and its counterpoise) 
to that of the centre of revolution of the polar axis, removed all end pressure of the 
declination axis, and supported all but a small fraction of these weights by anti- 
friction rollers. In this arrangement great steadiness is retained and freedom of 
moticn attained. An instrument combining these principles and carrying a 12-inch 
_ achromatic of 20 feet focus, has but about 12 cwt. of material (including the tele- 
scope) to be.moved; and this is effected by a force of about 1 lb. applied at the eye 
end. This instrument, contrasted with the 8-inch before mentioned, is (allowing 
for the difference in size) lighter in the proportion of about hundredweights to tons, 
The author, in conclusion, and aided by drawings, explained the general construction 
of an instrument of the German type which he had devised purposely for the pro- 
posed great southern telescope, and which construction had been selected by the 
Committee appointed by the British Association in reference to the same. . In this 
instrument a telescope of the proposed diameter (viz. 4 feet), and the other moving 
portions of the instrument, are calculated at 19,000 lbs., moved by a force of 20 lbs., 
applied at a radius of 5 feet; the other proposed construction, which was that of 
the prolonged polar axis, being estimated at 45,000 lbs. moving weight, and requiring 
750 lbs., or 374 times that of the author’s construction for its movement. 
On a Method of determining whether the Luminiferous Vibrations are Parallel 
or Perpendicular to the Plane of Polarization. By M. it’ Asst Moreno, 
Paris. 
By a truly extraordinary tour de force, such as we find no other example of in the 
history of mathematics applied to physics, M. Cauchy, starting from two angles deter- 
mined experimentally by Sir David Brewster, the principal angle of incidence and the 
