and Polarization of Heat. 155 
52. There were two distinct interposed plates employed for 
these experiments ; their thickness was such as to transmit the 
red of the second order of the Newtonian Scale, when viewed by 
polarized light, analyzed at right angles to the plane of polariza- 
tion. To shew that no appreciable difference existed in their 
power of stopping common or unpolarized heat, and to point out 
the accuracy of such determinations, I may quote the following 
experiment on the transmission of unpolarized non-luminous heat 
through the two plates. 
Plate with sides inclined 0° and 90° Plate with sides inclined 45° 
to Principal Section. to Principal Section. 
Mean 
1Bsid 18°.95 182 
18} 
17.9 175 
he 18.0 18 
181 j 
Cc 
is f 18.25 18+ 
The reduction is performed as in art. 20. These quantities 
were observed with the naked eye, and may therefore be con- 
sidered as coinciding in the two columns. 
58. In repeating these experiments with a single film of mica, 
which was alternately placed with its axis parallel or inclined 45° 
to the plane of primitive polarization, similar results were obtain- 
ed. With incandescent platinum, the result is of the most striking 
character ; under favourable circumstances, the needle moves 
through from 2° to 3° degrees, (a quantity, it will be recollected, 
of which a twentieth or a thirtieth part is capable of measurement 
by the improved method of observation), or even more, com- 
mencing the moment that the change in the position of the mica 
film is effected (which I generally perform with long forceps, so 
as to avoid the near approach of the hand to the pile). A few 
of the first experiments gave for the ratio of the effect on the 
pile in the two positions, with a single plate, 
u 2 
