Third Series.'— Refrangibiliti/ of Heat. 



187 



tensity of reflected light being made at intervals. The series 

 was then frequently reversed, and the mean results of the 

 going and returning series taken to allow for any change 

 which might have occurred in the intensity of the source. In 

 all cases an observation of verification was made and such 

 change allowed for. The dynamical effect on the galvano- 

 meter was observed and noted. 



In reducing the observations the following plan was 

 adopted. The intensity corresponding to the diagonal 14*5 

 inches being assumed = 100, the other intensities were re- 

 duced relatively to it, and projected, as explained in art. 60. 

 By this means different series of observations became at once 

 comparable with each other, and the beauty and regularity 

 of the curves thus formed, and the almost perfect identity of 

 those obtained on different days, and with different adjust- 

 ments, give a degree of confidence in the results which is ex- 

 tremely satisfactory. When from the nature of the heat the 

 effect was very small (as in the case of alum being interposed, 

 or the source being of low temperature), I have endeavoured 

 to supply the deficiency by multiplying observations, and the 

 uniformity of the curves thus obtained has been the test of 

 my success. Where this test has failed (as in the attempt to 

 work with heat of 212°), I have suppressed the results. 



Example. — Dark Hot Brass. March 31, 1838. 



After the observations made as now described have been 

 projected in the form shown, Plate IV. fig. 2, the diagonal 

 corresponding to the maximum rate of decrease of the in- 

 tensity was determined, for the purpose of deducing the index 

 of refraction. The following enumerations of the kinds of 

 heat employed, and the results derived from the several pro- 



