E AVIATION. 187 



mioa because he had observed that mica transmitted a 

 large proportion of non-humous rays. He devised means of 

 splitting mica into a large number of minute plates by 

 exposing it to the action of heat. By so doing he was 

 able to keep the mica in one mass, and yet to have that 

 split into a large number of plates, and this was tho 

 material employed which led to successful observations 

 being made on the polarisation of heat. Before illustrating 

 this I will point out the table of Principal Forbes' results, 

 which is as follows : 



FORBES' EXPERIMENT. 

 Refractive Index of Rock Salt. 



Heat from lime, mean value. . . .1*531 

 Heat passed through glass .... T547 

 Heat passed through alum . . . . 1 '558 

 Mean luminous ray 1 '562 



This proves in another way the identity of luminous and 

 non-luminous vibrations. By employing the angle of total 

 reflection with a prism of rock-salt, and measuring this 

 angle, Principal Forbes was able to find by simple geome- 

 trical principles the refractive index of rock-salt for dif- 

 ferent sources of heat. He first used the heat of lime, 

 then, after passing it through glass, he found a different 

 refractive index; and then when he passed it through 

 alum he got a third value, and from the mean luminous 

 ray he got a fourth value. You see the only difference is 

 that the lower the source of heat the more near it is to the 

 red end of the spectrum, or beyond the red end the greater 

 is the refractive index, which is in accordance with theory. 



I will now try to show you with this apparatus the 

 experiment on the polarisation of heat. Unfortunately 

 these instruments, which were employed by Principal Forbes, 

 and which have been lent by Professor Tait, are in an 

 extremely fragile condition, and are also covered with dust. 

 I hardly like to run the risk of injuring them by wiping 

 the dust off, and therefore I will use a slightly different 

 form of apparatus, which will answer the same purpose. I 

 have here two large prisms which have been lent by Prof. 

 Guthrie. By means of this large Nicol prism we can 

 get a polarised beam of light. This is called the polariscr, 



