168 Professor Forbes on the Polarization of Heat. 
polarized. When suitably modified, these rays are capable of in- 
terfering like those of light. 
6. The characteristic law of depolarization in the case of light 
holds in that of heat, viz. that the intensities in rectangular posi- 
tions of the analyzing plate, are complementary to one another. 
7. As a necessary consequence of the above, confirmed by ex- 
periment, heat is susceptible of circular and elliptic polarization. 
8. The undulations of obscure heat are probably longer than 
those of light. A method is pointed out for deducing their 
length numerically. 
78. Of the evidence for these conclusions I have enabled the 
reader to judge, by specifying numerical results. But I must 
farther add, that all the principal conclusions were arrived at by 
the indications of the galvanometer, observed by the naked eye, 
including the chief phenomena of depolarization. Since I thought 
of the method of magnifying the divisions (described in (5),) I had 
little else to perform than the agreeable task of verifying and de- 
fining my first conclusions. 
EpInBURGH, 
19th January 1835. 
