8 REPORT—1840. 
and Polarization of Heat,’’ read to the Royal Society of Edin- 
burgh, Jan. 5th and 19th, 1835, and published in their Trans- 
actions, vol. xiii.; also in the London and Edinburgh Journal 
of Science, vol. vi. 
The first section contains an account of various experiments 
with the thermo-multiplier. The principal object was to verify 
the several points already stated, and especially to determine 
the degree of accuracy of the instrument. From a comparison 
of its sensibility with that of air-thermometers, the author 
concludes that 1° of deviation of the needle corresponds to an 
effect indicated by about =,th of a centigrade degree, Without 
increasing the dimensions of the instrument, by which its sen- 
sibility would be impaired, he has been enabled, by the adapta- 
tion of a small telescope, readily to measure ;pth of its degrees ; 
that is, about >3,th of a centigrade degree. 
One of the most interesting points to which the author 
directed his attention, was the possibility of detecting heat in 
the moon’s rays. These rays, concentrated by a polyzonal lens 
of 32 inches diameter, and acting on the thermo-multiplier, 
gave no indication of any effect; so that Prof. Forbes con- 
siders it certain that, if there be any, it must be less than 
sonssoth of a centigrade degree. 
He repeated Melloni’s experiment of the refraction of heat 
by a rock-salt prism, and was enabled to obtain some approxi- 
mate quantitative results, giving the index of refraction for 
heat in this substance, which was a little less than that for 
light. 
In the course of his second section he describes further 
experiments relative to the question discussed by Melloni, of 
the separation of the effects due to heat and light, especially 
the peculiarity (before mentioned) attending green light: he 
tried flames variously coloured with salts—giving red, yellow, 
green, and blue light; but found the proportions of rays trans- 
mitted by alum, glass, and rock salt to be nearly constant for 
each substance. 
To this part of the subject Prof. Forbes again directed his 
attention, in a later series of experiments, in which ke has 
obtained numerical results of the highest value. These are 
detailed in the last part of his third series of Researches on 
Heat, read before the Royal Society of Edinburgh, April 16, 
1838, and published in the Transactions of that body, vol. xiv. 
To the earlier portion of this memoir we shall refer, under 
another division of this report. 
The third section relates to the Index of refraction for heat 
of different kinds, as compared with that for light in the same 
