1886.] Light reflected at nearly Perpendicular Incidence, 287 



The following is a summary of the results obtained at this time : — 



Face of Prism (I). 



Lord Kayleigh. Mrs. Sidgwick. 



Aug. 4.... 0-0411 Aug. 4 0-0405 



„ 5.... 0-0413 „ 5.... 0-0413 



„ 7.... 0-0403 . — 



„ 9.... 0-0413 ...... — 



Mean.. 0-0410 Mean.. 0*0409 



Final mean 0*04095. 



Since this number is very nearly the same as that (0*04119) due to 

 the disk alone, we see that the result is scarcely at all dependent upon 

 the correctness of the assumed effect of the oblique plate. 



It now remains to make comparison with the reflection as given by 

 Fresnel's formula, viz. :— 



sin 2(0-0') ( 

 sin2(0 + 0')' K J 



where sin 0/sin 9'=-fi, and fi is the refractive index. 



The index was determined in the usual way from the angle of the 

 prism (i) and from the minimum deviation (D). The value of i was 

 found to be 9° 50'. The minimum deviation of soda light on one side 

 was 5° 5', and on the other 5° 4J'. Thus D = 5° 4j', and 



^ = sini(D + = 1 . 5141 _ 

 sin -ii 



The angle of incidence (0), which was the same in all the observa- 

 tions With the various reflectors, was measured by determining the 

 angle (20) between the incident and reflected ray. This measurement 

 does not require great precision, for a change of a whole degree in the 

 value of would alter the reflection about 1 per cent. only. I 

 found — '■ 



= 13° 52'. 



With these values of and /*, we find — 



= 0*04514, 



sin 2 (0-0') _ 



sin 2 (0 + 0') 



about 10 per cent, in excess of the reflection actually observed. 



In order to satisfy myself that the deficiency of reflection was real 

 and permanent, this prism was remounted after a thorough cleaning, 

 and further observations were taken, as summarised in the following 

 table. The revolving disk was the same as before : — 



