202 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



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

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

 art. 53. For the pm-pose of ready comparison I have calculated the following 

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

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

 of art. 53 : 



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

 fiexm-e, and consequent values of the indices of refraction of the most ahundatit 

 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. 



f Omitted in the final reductions on account of the irregularity of the observations. 



