55i! Prof. Forbes*s Researches on Heat Second Series, 



Rock-salt, with a thin coating of varnish. 

 C Polished rock-salt. 

 ^ Glass. 

 (^Alum. 

 The three last substances (so different in their diathermancy) 

 were nearly equal in their reflective power for dark heat (from 

 brass about 700"). The above order did not, however, appear 

 to be changed for heat from incandescent platinum, except 

 that glass seemed to stand above alum and even salt. In a 

 general way, we may consider the measure of metallic reflec- 

 tion to be from two to three times as great as that from mica 

 split by heat, which is also very superior to a single surface 

 of mica. Glass, salt, and alum seem to reflect but a third or 

 a fourth part, or even less, of that furnished by the laminated 

 mica. 



I did not, as I have said, stop to prosecute these experi- 

 ments; they clearly showed that of the substances which I 

 tried, mica split into thin films afforded the most copious re- 

 flection (next to the metals), and this was the very substance 

 which from the first I had employed. They likewise satis- 

 factorily showed the cause of the failure in former attempts to 

 polarize by reflection, seeing that, for dark heat, glass is 

 almost the worst reflector that could have been used, and 

 as it likewise absorbs almost all the heat, transmitting very 

 little, the effect of secondary radiation is increased. Hence the 

 difficulty experienced by Professor Powell and others in 

 getting any results at all* before the thermo-multiplier wa& 

 devised, and the failure of the attempts of Signor Nobili of 

 Florence even with its aidf . The last-named eminent philo- 

 sopher failed also in obtaining traces of polarization by me- 

 tallic reflection, which was not to be wondered at, as on an- 

 other occasion we shall be able to make to appear. 



The form of apparatus which I have more recently em- 

 ployed for experiments on polarization by reflection was sug- 

 gested to me by the present astronomer royal, Mr. Airy, after 

 the publication of my first paper. It is represented in section 

 in the figure in the opposite page. 



AB and CD are two reflecting surfaces of mica fixed to 

 blocks G and H ; the former of which is attached to a board 

 TU, carrying the lamp or source of heat S, and revolving in 

 a horizontal plane round T as a centre ; — the latter (H) is per- 

 manently fixed relatively to the pile P, provided w^ith its conical 

 reflector. The surfaces AB, CD are parallel, and make angles 

 of about 5Q^ with the horizon ; consequently the heat falling 



* Edinburgh Journal of Science, N. S., vols. iii. and v. 

 t Bibliotheque Universelle, Sept 1834. 



