556 Prof. Forbes's Researches on Heat. Second Series, 



heat from an Argand lamp, whilst that from incandescent 

 platinum was more polarizable than either. The following 

 results were obtained on the 12th March, 1836. The source 

 of heat was in all cases at a distance of six and a half in- 

 ches from the centre of the first reflecting plate, and the 

 whole length of the dotted line PFES was about sixteen 

 inches. The reflecting plates were composed of ten or twelve 

 laminae of mica, split with a pen-knife, and the plane of re- 

 flection was perpendicular to the principal section of the 

 mica. 



Rays out of 100 polarized. 

 Argand lamp without reflector, .... 55 

 Dark heat, from brass about 700°, ... 61 

 Incandescent platinum, 65 



A mica-plate placed between the two reflecting surfaces in 

 the figure, perpendicularly to the reflected ray, is capable of de- 

 polarizing the heat, as in the case of heat polarized by trans- 

 mission (17th December, 1835). The fact is simply men- 

 tioned here, as we do not at present resume the subject of de- 

 polarization. 



I made some experiments, with a view to the determination 

 of the maximum polarizing angle for heat, with a more conve- 

 nient apparatus than the one above described. 



It is well known that the following law holds for polar- 

 ized light. When lights polarized in any plane^ is reflected 

 from a refracting surface AT the polarizing angle for that 

 surface^ it is "wholly polarized in the plane of incidence. If 

 it be incident at a SMALLER angle than the polarizing anglcy 

 the refected light is polarized in a plane lying on the farther 

 side of the plane of incidence from the plane of primitive 

 polarization. If it be incident at a GREATER angle than 

 the polarizing angle, the plane of polarization will be on the 

 same side of' the plane of incidence as at first. Now, this I 

 have fully verified in the case of heat. Having polarized heat 

 by transmission through a mica bundle, in a plane inclined 

 + 45° to the plane of reflection, which it subsequently under- 

 went at the first surface of a thick mica plate, I examined its 

 state of polarization by another similar mica bundle interposed 

 between the reflecting mica and the thermal pile. I' found 

 that at great incidences the plane of polarization was on the 

 same side of the plane of reflection as at first, whilst at smaller 

 incidences it was thrown to the opposite side. I varied the 

 incidence until the plane of polarization coincided with the 

 plane of reflection, when I concluded that I had reached the 

 polarizing angle. This was found by the quantity of effect 

 when the plane of analysation was inclined -f- 45° and — 45° 

 to the plane of reflection. With dark heat, from brass at 



