372 Dr. Draper's Telescope. 



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/' inter- 

 locking" 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, runs 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 con- 

 struction. 



The mode of giving a parabolic figure finally preferred by 

 Dr. D., is that of " local 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. Leon 

 Foucault, at Paris, was employed by hand, but has been prac- 

 tised 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 <y 2 Addromedce ; 

 while so great is the light- collecting power of 154 inches, that 

 the companion of Wega can be perceived even with the un- 

 silvered 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 

 ao'oVoo °f an i ncn — 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 consequently 

 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 



