Notes. 



419 



With a diffraction grating the formula which connects the angle 

 of deviation of the light with the wave-length is as follows — 



X 



Sin V = 



(1) 



where 6 = the angle of deviation, 

 \ = wave-length, 



A = the centre to centre distance of the lines of the diffrac- 

 tion grating. 

 ]\ T ow if the lines on the scale E are ruled parallel to the central 



Fig. 53. 



line X X, their distance from the central line X X divided by the 

 radius of the circle on which the arm C swings is the sine of the 



angle of deviation, and is therefore proportionate to - or 



Sin ° = E = A * * * W 



when s = one of the divisions of scale E, and E = radius of circle. 

 Tenths of an inch have been selected as being a convenient 



