the received Theory of Newton's llings. 457 



the system however being still bright, with the homogeneous 

 light of a lamp of alcohol and chloride of sodium * for the 

 angle of incidence 39° 41' (the angle of refraction r from glass 

 to air), I found, by taking in each case the mean of repeated 

 measurements, the radii to be— 



Reflected Light. 

 Radii of the 



first, second, third, fourth bright ring. 



0*1244" Engl. 0*1602" 0'1900" 



first, second, third, fourth dark ring. 



0-1021" Engl. 0-1439" 01765" 0*2040" 



These values however need a sensible correction, inasmuch 

 as the radii of the rings, on account of the refraction of the 

 upper plate, a quarter of an inch thick, appear smaller than 

 they really are at the upper limit of the air-layer. The mean 

 exponent of refraction necessary for the ascertaining of these 

 corrections, I obtained according to the method of Prechtlf 5 

 from a series of ten measurements. The microscopic lens 

 being 2*91 English inches distant, the following corrections 

 are obtained : — 



Reflected Light. 



Corrections for the 



first, second, third, fourth bright ring. 



0-0070" 0-0091" 0*0106" 



first, second, third, fourth dark ring. 



0-0056" 0-0081" 0-0099" 0'OllS" 



These corrections being applied to the radii found above, 

 and the radius of the first bright ring being calculated from 

 the second by means of the proportion 1 : V3, we obtain, 

 finally, — 



Reflected Light. 



Radii of the 



i * * ; > 



first, second, third, fourth bright ring. 



0-0758" 0-1314" 0-1693" 0'2006" 



first, second, third, fourth dark ring. 



0-1077" 0-1520" 0-1864" 0-2153" 



* This light is not completely homogeneous, but a mixture of yellow 

 and violet, as I found by decomposition with a prism; so that it deserves 

 to be called orange rather than yellow. It is however sufficiently homo- 

 geneous for the purpose to which it has been here applied. 



f Practical Dioptrics. Vienna, 1828, p. 127. 



