392 OPTICAL PROJECTION 



sensitive thermopile, and sensitive galvanometer, must be 

 employed. The needle of the latter may either travel over a 

 horizontal glass dial, projected in the vertical attachment, or 

 a pencil from an oscillating mirror may be employed. 



With glass lenses and a disulphide prism, the demonstra- 

 tion must be comparatively rough. The maximum heat will 

 now be generally found in the red itself, and the galvano- 

 meter with perpendicular glass dial (fig. 220) can be made 

 sensitive enough to project all that can really be shown with 

 such apparatus. 



It is usual to employ two lanterns or two nozzles, one 

 giving the spectrum in which the thermo-pile is moved, and 



FIG. 216. Examining the Spectrum 



the other projecting the galvanometer movement above it ; 

 if two nozzles are employed on one concentric lantern, this 

 must be so adjusted that the common radiant is at the proper 

 focal distance for each of the two sets of condensers. On the 

 whole, a second lantern for the galvanometer is most con- 

 venient. It is also convenient to have at disposal two 

 galvanometers, one very different from the other as regards 

 sensitiveness. (Also see next section.) 



232. Identity of Light and Radiant Heat. By similarly 

 using a thermo-pile, the double refraction, polarisation, and 

 depolarisation of heat rays are readily shown. If luminous 

 heat rays are employed, the thermo-pile has simply to be 



