188 LECTUBES TO SCIENCE TEACHERS. 



and the next is called the analyser \ and when the light is 

 passed through this it is in a peculiar condition it is said 

 to be polarised ; but if after that I turn this round its axis 

 in different directions, I shall find that in some positions it 

 will cut off all the light, but in others it will allow a con- 

 siderable amount of light to pass. If our adjustments 

 were perfect, it would now have totally disappeared, but 

 there is a difficulty in preventing a slight reflection from 

 the interior of this prism. Now I wish to show the same 

 phenomena with respect to heat ; but before doing so I 

 shall show you another experiment which we also have 

 with regard to light. When we have turned this round 

 until we get a total obscuration of light, or very nearly so, 

 I will pass between the polariser and the analyser a piece 

 of crystal mica, and that has the effect of altering the 

 condition of this ray and depolarising it so that it is again 

 at liberty to pass through this second prism. You see the 

 colours there, showing that the light is again passing 

 through, and by rotating the mica I can get different 

 colours. In the case of heat it will be most important 

 to employ those colours which are of a deep red, and then 

 we shall be more likely to get non-luminous rays trans- 

 mitted. You see first the light is polarised by this prism, 

 then on examining it with the second prism we find that 

 in certain positions it is allowed to pass by transmission, 

 whereas in others it is stopped out entirely ; but when it is 

 stopped out we get the light passing through the piece of 

 mica again Principal Forbes employed a non-luminous 

 source of heat, viz., boiling water ; but since we are un- 

 able to employ the same delicate means which he had, on 

 account of not having these mica plates, we shall have to 

 use a luminous source of heat and cut off the luminous 

 portion by means of a solution of iodine in bisulphate of 

 carbon ; that will cut off entirely the luminous rays and 

 leave only the dark ones, because this solution is absolutely 

 opaque and does not allow any luminous rays to pass 

 through it, but it permits the ultra red rays to pass through 

 it with great facility. Previously to placing that in the 

 beam I will place this thermopile as nearly as possible in 

 the focus of the rays. You see a deflection produced in the 

 galvanometer by the luminous rays falling upon it. I will 

 now cut them off, leaving only the dark field. The solution 



