DRAPER’S TELESCOPE. 65 
A full trial was given to no less than seven machines, on the principles 
employed by Lord Rosse and Lassell, with modifications of his own. ‘The 
prime mover, called the “ foot-power,’’ was avery ingenious contrivance, in 
which very little force is lost in overcoming friction, and which is frequently 
employed in America for dairy use. Dr. D. himself generally walked in his 
own, and has travelled some days, during five hours, more than ten miles. It 
consists of an endless band of short transverse boards or “ treads,’ interlocking 
so as to form a platform to tread upon, which will not yield downwards on its 
upper side, but hangs loose in the return half beneath, and passing over wheels 
and rollers at either end. This succession of boards, having one end a little 
higher than the other, rams downwards as soon as a weight is placed upon it, 
and communicates motion to a large wheel on the axle of the one over which it 
turns, and through it to any connected machinery. Being placed between a 
handrail on either side, it offers the appearance of a little narrow bridge, as over 
a ditch, composed of transverse boards, on which the mover may walk all day 
without getting a step forward. It is, in fact, a species of treadmill, of a much 
more pleasant construction. 
The mode of giving a parabolic figure finally preferred by Dr. D. is that of 
“‘loeal retouches,” in which the edge of a spherical mirror is flattened, or, which 
he thinks preferable, the centre is bored out deeper, by appropriate polishers of 
curvatures differing slightly from that of the original tool on which it was 
wrought. This method, as invented by M. Léon Foucault, at Paris, was em- 
ployed by hand, but has been practiced by Dr. D. with suitable machinery, and 
with excellent results ; his great specula, thus finished, bearing a power of 1200, 
and dividing the celebrated test-pair 7? Andromede; while so great is the 
light-collecting power of 154 inches, that the companion of Wega can be per- 
ceived even with the unsilvered surface; some portions of the moon are even 
more visible than after silvering—a hint worth notice. When silvered, the 
quantity of lunar light is so overpowering as to impair for a long time the vision 
of an eye placed at the focus. Several modes of silvering were tried by Dr. D., 
some devised by himself. Foucault’s proved uncertain in its results; that of 
Cimeg, with tartrate of potash and soda, for looking-glasses, modified so as to 
fit the silver for being polished on the reverse side, he found superior to any, 
and in using it “never on any occasion failed to secure bright, hard, and in 
every respect perfect films.” Their thickness is about 559595 of an inch—nearly 
the same with that of gold-leaf of equal transparency—the sun appearing 
through the silver of a light-blue tint. Variations in its thickness are conse- 
quently only small fractions of that fraction, and of no optical moment whatever. 
It tarnishes quickly if exposed to sulphuretted hydrogen—a defect which has 
been avoided in the English process—and it may be split up into fissures by 
damp; but heat does not affect it, and it is generally very enduring. “I have 
some,” the doctor says, “which have been used as diagonal reflectors in the 
Newtonian, and have been exposed during a large part of the day to the heat of 
the sun concentwated by the 15}-inch mirror. These small mirrors are never 
covered, and yet the one now in the telescope has been there a year, and has 
58 
