414 C. Barus — Methods in Reversed and 



of 4 fringes were tested, as these required an adequately large 

 displacement of the micrometer, which was moved both for- 

 ward and backward. The results (omitting details) were : 



Mean rfe 

 displacement 

 No. of fringes per fringe 



(increasing) 3 •0088 cm 



(increasing) 4 ■l)080 cra 



(decreasing) 4 -oo^S " 1 



The mean of the three results is Be = •008 cm per fringe. The 

 individual data were not smooth, because the micrometer plaped 

 between the mirrors M and IV, is in an inconvenient position 

 for manipulation. The different sets of values, moreover, cor- 

 respond to different adjustments and therefore to slightly 

 different values of a — (j>. As an order of values only is 

 wanted, it was not considered worth while to remedy the 

 deficiencies. 



In accordance with the equation given, if he = •008 cm , 

 \ = 58-9 X 10- 6 cm., 2 = 20° be inserted, 



A cos' #„ 

 a- 6 = -= — : — ~ =-0095 radians = 54°. 

 ^ 28e sin t 



The adjustment is thus about half a degree out of symmetry, a 

 result which in case of improvised apparatus is inevitable and 

 moreover without significance in the precision of the method. 



9. Prism Method. Reflection. — The grating G was now 

 removed and replaced by a silvered prism (as shown in fig. 9). 

 A small prism angle, <£, is essential (c£ = 18°, about), as a large 

 divergence of rays would not be accommodated on the inter- 

 ferometer, fig. "6. The fringes were found without difficulty, 

 in the second order, the arc lamp being used. They are also 

 easily distorted, if the edge of. the prism is not parallel to the 

 rulings of the grating. In such a case the symmetrical arrow- 

 shaped forms become one-sided and, as it were, curved or 

 faintly fringed beyond the limits of the strip. To get the best 

 adjustment, the lamp should shed about the same amount of 

 undeviated light from both faces of the prism, on a screen 

 temporarily placed behind it. The illuminated strips on the 

 grating must coincide to the eye, while making the fore and 

 aft adjustment. Finally the grating is to be slowly rotated on 

 the axis normal to itself, until fringes of satisfactory shape and 

 size appear. Naturally this is done through the telescope and 

 a readjustment of the longitudinal axes of the spectra is neces- 

 sary after each step of rotation. Fringes so obtained are as 

 good as those obtained by any other method. 



The range within which the fringes are sharp is small, not 



