202 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



Source of Heat. 



Locatelli; direct, . . 



„ with Alum, . 



™„-™ Window-Glass, 



Opaque Glass, 



Mica, 



Incandescent Platinum, 

 Ditto, with Glass, . . 

 Ditto with opaque Mica, 

 Brass at 700°, . _ . . 

 Ditto with clear Mica, 

 Mercury at 450°, . . 



Diagonal corresponding to Point of contrary flexure of 

 Curve. 



Before March 21. 



15.47 15.50 15.60* 

 15.64 



15.51 15.47 



15.44 15.42 



Since Match 21. 



15.54 



15.79 



15.70 



15.75 



15.60 



15.62 



15.64 



15.62 



15.45 



15.62 1 



16.52 



15.47 

 15.73 

 15.60 

 16.67 

 15.62 



15.67 



15.47 

 16.55 

 15.52 



16.45 



15.45 



72. From these numbers we can of course compute the corresponding angles 

 of incidence, and thence the value of the index of refraction by the formula of 

 art. 53. For the purpose of ready comparison I have calculated the following 

 table, which gives the angles of incidence, and consequently the indices of refrac- 

 tion, corresponding to different values of the diagonal computed by the formula 

 of art. 53 : 



Diagonal 

 ab. 



Angle of Inci- 



Angle of Total 



Index of Re- 



dence 



Reflection 



fraction 



= «. 



= /3. 



= fi. 



Inches. 



o / 



O / 





16.0 



— 1 25 



40 66 



1.627 



16.1 





40 37 



1.636 



15.2 



— 32 



40 20 



1.546 



16.3 





40 3 



1.564 



15.4 



-hO 21 



39 47 



1.563 



15.6 





39 30 



1.572 



16.6 



+ 1 16 



39 12 



1.682 



16.7 





38 56 



1.592 



15.8 



-f-2 11 



38 37 



1.602 



15.9 





38 20 



1.612 



16.0 



+ 38 



38 4 



1.622 



73. We have the following mean values of ab for the points of contrary 

 flexure, and consequent values of the indices of refraction of the jnost abundant 

 rays in each source : 



* This observation was made with a very contracted diaphragm ; the readings therefore were very 

 small. It is omitted in the final reductions. 



■j" Omitted in the final reductions on account of the irregularity of the observations. 



