448 Professor Forses’s Researches on Heat. 
a micrometer does the limb of an astronomical circle.* The re- 
sult of this application of optical power is, that equally accurate 
conclusions may be drawn within small limits of deviation, as if 
the deviations were greater, and observed in the ordinary way. 
This is important on several accounts, 1. The instrument is not 
liable to those derangements which follow from exposure to con- 
siderable heat,—derangements difficult to allow for, and which I 
have not succeeded in obviating. 2. The value of the degrees is 
more nearly uniform, and less liable to abrupt change ; so that (as 
will presently be seen) within the narrow limits under which I am 
accustomed to operate, the deviations are almost as the forces. 
3. The motions of the needle being much more speedy and cer- 
tain within small ranges (particularly in its return to zero) much 
time is saved, and the consecutive observations are more accurately 
comparative. 4. By the use of the telescope all parallax in read- 
ing is avoided, and if a diagonal eye-piece be employed, the posture 
is much less fatiguing than in any other method of observation. 
6. Besides using this optical contrivance, I have increased the 
delicacy of the instrument, by adding a conical reflector (seen at 
M, Fig. 1), so as to concentrate nearly parallel rays upon the sur- 
face of the pile. This contrivance is not my own. I saw one 
in an instrument made on the model of M. Nosixt’s, in the 
possession of M. Quere.er, at Brussels, in 1832, which was the 
first multiplier I had seen. This reflector increases the sensibi- 
lity of the pile to heat from a given source seven or eight times, 
or in a proportion not very different from the area of its aperture 
* References to Fig. 1—P, the thermal pile. KI, LH, the wires conveying 
the electricity from the pile to the galvanometer. G, the galvanometer card, over 
which the needle traverses. EF, the cover of the galvanometer, which has a plane 
glass top. BC, a telescope, with a diagonal eye-piece, having an additional object- 
glass at D, in order to give distinct vision of the galvanometer-needle and the scale 
(see First Series, art. 5). A, the telescope-bearer, moving round a point concentric 
with G M, the trumpet-mouthed reflector, applied to the pile (art. 6, below). 
