THIRD SERIES— REFRANGIBILITY OF HEAT. 



203 



Source of Heat. 



Locatelli, direct, . . 



-with Alum, . 



, Window-Glass, 



^ — opaque Glass, 



,^ _-™_ Mica, 



Incandescent Platinum, 

 Ditto with Glass, . . 



opaque Mica, 



Brass at 700°, . . . 

 Ditto with clear Mica, 

 Mercury at 450°, . . 



Mean Luminous Rays, 



ah. 



15.49 

 15.76 

 15.65 

 15.71 

 15.61 

 15.50 

 15.66 

 16.62 

 15.45 

 15.55 

 15.50 



16.8 



1.571 

 1.598 

 1.587 

 1.593 

 1.583 

 1.572 

 1.588 

 1.584 

 1.568 

 1.677 

 1.572 



1.602 



74. In the following table I have given the mean results of the different se- 

 ries of observations on which the above conclusions are founded, and from these 

 numbers I have projected the curves exhibited in Plate XII, the dots correspond- 

 ing to the numbers here given, and the mode of projection being that already 

 explained : 



Source op Heat. 







Values of the 



Diagonal 



ab. 







14.5 



15.0 



16.25 



15.5 



15.75 



16.0 



16.25 



16.5 



Locatelli; direct, .... 



100 



93 



80 



60 



41 



30 



2 



18 



~ with Alum, . . . 



100 



91.6 



84.5 



71.5 



51 



39 



24 



18.5 



.^.,ror«r.r. f, f,. ^V i HdO W-Gl flCiq 



100 



Q.S 



84 



fifi 



47 

 55 



33 



23 /» 



18 



opaque Glass, . . 



100 



97.5 



89 



75.5 



42.6 



26 



22.6 



~~ Mica, , . 



100 



94 



82 



67.5 



48.5 



32.5 



23 



20 



Incandescent Platinum, . . 



100 



89 



75 



58 



41 



80 



22 



18 



Ditto with Glass, .... 



100 



88 



77 



62.5 



42.6 



30.6 



23 



17.5 



opaque Mica, . . . 



100 



92 



78 



63 



46 



30 



22 



18 



Brass at 700°, 



100 



84 



69 



61 



35.5 



25 



20 



15 



Ditto with clear Mica, . . 



100 



85 



71 



62 



33 



26 



13 



11 



Mercury at 450°, .... 



100 



92 



77 



57 



42 



29 



22 



13 



75. When we compare the preceding results, obtained with a rock-salt prism, 

 with those for light, we find that the received index of refraction for that sub- 

 stance would give to heat a higher degree of refrangibility than light, a result 

 contrary to all probability. This, however, is not confirmed by du-ect experiment. 

 Placing a bright small source of light at S (Plate XIII. Fig. 1), and a screen at^j*, 

 I find the index of refraction for the most luminous rays to be higher than that 

 of any of the above kinds of heat, being at least 1.602, corresponding to a diagonal 

 ah =15.8 inches, as I have given it above. By two series of results derived from 

 a very small oil flame (without wick), I got 15.87 for the diagonal both times ; 

 and from the Locatelli-lamp (which on account of the size of the flame forms a 

 better standard of comparison with the experiments on heat) 15.76 ; so that I 

 consider 15.8 as a fan- representation of the case of light. 



76. Yet it is quite certain that the index of refraction of the rock-salt used 

 is really much below 1.60. A single experiment with Dr Wollaston's instru- 



