TRANSACTIONS OF THE SECTIONS. 11 
tion of the most refrangible red and the orange and yellow rays only passed ; and the 
ammonia sulphate of copper, which absorbed all but the most refrangible portion of 
the spectrum. Thus were obtained the means of isolating, with a tolerable degree of 
purity, the calorific, the luminous, and the chemical spectra. Having several designs 
cut out of white paper, these were placed on copper plates, and being covered with 
the above bottles of fluids, placed in the sunshine, After remaining exposed for dif- 
ferent periods, at different times, from half an hour to three hours, they were brought 
from the light, and the plates placed in the mercurial vapour-box, and subjected to its 
influence, In no instance did any impression appear on the plates which were placed 
under the blue or yellow fluids, but in every case most decided impressions on those 
plates which were subjected to the influence of the red rays. Indeed, in some cases the 
impressions were beautifully visible without the use of mercurial vapour. It does 
therefore appear, when we take into consideration, besides the above facts, also the 
fact which has been admitted, that artificial heat at least accelerates this molecular 
change, that an amount of evidence has been obtained in favour of the hypothesis of 
calorific disturbance, superior to the supposed evidences in favour of the absorption 
and radiation of any other solar emanation, Mr. Hunt caused the prismatic spec- 
trum, which was kept stationary by means of a heliostat, to fall upon unprepared 
copper-plates, and kept the plates under its influence for some time. On exposing 
these plates to mercurial vapour, the space over which the luminous rays fell was, to 
a certain extent, protected from the vapour, whilst the space which corresponded with 
the maximum calorific rays was thickly covered with it. Mr. Hunt varied his expe- 
riments with the spectrum, but a positive action was detected only in the thermic rays. 
From another series of experiments made with washed and unwashed plates, Mr. 
Hunt concluded that organic matter is not the cause of these images, but that the 
effect is due either to some disturbance of the latent caloric, which produces a mole- 
cular change, or to a thermo-electrical action, which it is difficult to understand. Had 
the effect heen due, as M. Fizeau has stated, to slight layers of organic matter of a 
volatile nature, it appears natural to suppose that these mysterious images would 
have been found only on the very surfaces of the plates. Now this is far from being 
the case. These images are often found to be impressed to a great depth into the 
metal. Mr. Hunt in many cases removed several surfaces of copper, and yet had 
been able to revive the images. He possessed copper plates rendered useless by the 
impressions, which he has in vain endeavoured to remove. 
—— 
On polishing the Specula of Telescopes. By Dr. GREENE. 
Dr. Greene apologized for presuming to speak of his humble efforts after the splendid 
achievements of the noble President of the Association in the same department, by 
stating that few possessed the means, the perseverance or the courage to attempt any 
thing even approaching the gigantic instruments already constructed, and now, on 
even a larger scale, in process of construction, by Lord Rosse, whilst many an ama- 
teur would be delighted to amuse himself at a small expense in that delightful path of 
science, 
Dr. Greene then described the construction and action of his machine, which he 
illustrated on a small working model of only one foot in length, having then a mirror 
of one-inch aperture on its polisher. The doctor dwelt upon the greater convenience 
of using a machine turned by hand with a winch where the axes of the wheels are 
horizontal, than where they are vertical, as in Lord Rosse’s, which is driven by a 
steam-engine. But the principle of the machine so nearly resembles that far more 
perfect machine of his lordship’s, that by some few alterations it may be made identical 
with it in its mode of action. The general principle of the machine is this: a crank 
sets in motion a bar to which the mirror is attached, and which pushes it backwards 
and forwards over the polisher; while another crank, moving with a different velo- 
city and through a different space, acting on the other bar in a direction at right 
angles to the former bar, continually deflects it from a rectilineal into a curved path 
over the surface of the polisher, which is constantly revolving slowly in one direc- 
tion, whilst the mirror is made to revolve slowly in the opposite direction so as con- 
tinually to change the portions of both that act upon each other, It is a remarkable 

