Non-reversed Spectrum Interferometry. 409 



7. Compensator Measurements. — A. With the object of 

 testing the interferometer under a variety of conditions, 

 measurements were made with a numher of different compen- 

 sators and the experience obtained may be briefly given here. 

 The first of these was a very sharp wedge, such as may be 

 obtained from ordinary plate glass. The piece selected, cut 

 from an old mirror, on being calipered showed the following 

 dimensions : 



Length, 5 cni ; thickness at ends, -375 and -ZW m . 



Hence the angle of the wedge is a = '0016 radians or about 'l . 

 No difficulty is experienced from the deviation of the rays for 

 so small an angle, though sometimes the fringes are unequal 

 and the lines presumably curved. This wedge was attached to 

 a Fraunhofer micrometer, moving horizontally parallel to the 

 wedge, and the normality of the rays passing through the glass 

 was found by rotating it around an axis perpendicular to the 

 rays, until the direction of motion of the fringes was reversed. 

 In view of the small angle a and the micrometric displace- 

 ment, it was easy to count single fringes, or fractions as far as 

 about 1/30 of a fringe, even though the beam traversed 

 the glass twice. In the first experiments the data of the hori- 

 zontal displacement, r, of the wedge, were found for successions 

 of seven fringes. From the mean value of 8 such sets, 

 r = ^OOS "* and the displacement per fringe would be 

 Sr = -0287 cm . 



In another series made with, care as to the normal adjust- 

 ment, the horizontal displacement, r, of the wedge for succes- 

 sions of 11 parallel fringes was taken. Again omitting the 

 individual data, the mean displacement was found to be 

 r = -3014: cm , whence per fringe, 



8r= -0274 cm . 



This difference from the preceding result shows that extreme 

 care must be taken in placement. 



If x be the distance from apex of the wedge, its thickness is 

 e = xa, or per fringe he = a8x = aSr. The index of refraction 

 was found to be /* = 1*526 by total reflection. Thus without 

 correcting for dispersion, 



2{fi— l)8e = \ 



and with the above values 



10- 6 X 5-893 nnnn 



a = = "0020 radians. 



2 X -526 X '028 



This is larger than the calipered value, because the rays go 



