Intelligence and Miscellaneous Articles, 



4,77 



in the spectrum corresponding to the green light of thallium. They 

 give the ratio of the intensity of the reflected to the incident light 

 for two prisms — one of crown, and one of flint glass — the non- 

 reflecting surfaces of which were blackened with soot. The results, 

 each the mean of twelve experiments, are contained in the follow- 

 ing Table : — 



i. 



R. 



Crown glass. 



Flint glass. 



o 



30 

 40 

 50 

 55 

 60 

 65 

 70 



0055 

 0072 

 0104 

 0133 

 0174 

 0-231 

 0-293 



0070 

 0084 

 0120 

 0-161 

 0-203 

 0-254 

 0-327 



For a comparison with Iresnel's formula a more exact determi- 

 nation of the indices of refraction is needed. Their values as de- 

 termined from the deflections by the prisms cannot be used for this 

 purpose, since Seebeck has shown that the density, and therewith 

 also the exponent of refraction of the glass-surface, may be consi- 

 derably altered by the grinding ; hence the calculation is based on 

 the values obtained from the tangent of the polarization-angle. 

 This and the principal azimuth were determined for both prisms by 

 aid of the Babinet compensator, and gave the values stated under i x 

 and A in the next Table ; n is the value found for the line E from 

 the minimum of deflection. 



Crown Glass. 



^ = 56°25'-5 ; n'=V507. 



A= 1°20\ 



w=l-5275. 



I. 



Observed. 



Calculated. 



A. 



o 



30 



0055 



0059 



-0004 



40 



0072 



0073 



-0001 



50 



0104 



0114 



-0010 



55 



0133 



0141 



-0-008 



60 



0174 



0179 



-0005 



65 



0-231 



0-229 



+0002 



70 



0-293 



0-302 



-0009 



