430 Prof. Carl Barus on Iuterferometry 



or the corresponding equation in terms o£ N c . The trend 

 data for AN C agree fairly well with observation, except at 

 the J) line, which difference is very probably referable to the 

 properties of the glass, since the grating was not cut on 

 light crown. 



The number of fringes between the D and E lines now 

 comes out plausibly, being less than 4380"/104" = 42. It is 

 difficult to count these fringes without special methods of 

 experiment ; but the number computed is a reasonable order 

 of values, about 25 to 30 lines being observed. 



Some estimate may finally be attempted as to the mean 

 displacement of mirror SN per fringe, between the D and E 

 lines. As their deviation is #=1° 13' and the displacement 

 from D to E AN C , 



* N _ AN C _^AN _x* L AN c 



0l{d6ldn)~~ dn 2D cos AN C ' 



if the value of dd/dn for the D line be taken. Thus 



SN > X 2 /(2D cos 6) =X 2 /D sin 20, nearly. 



Hence SN is independent of the thickness, e, of the plate of 

 the grating, as I showed * by using a variety of different 

 thicknesses of compensator. Since 



X= 000059 cm., D = '000351 cm., SN>'00031 cm. 



The values found were between '00033 and *00039, naturally 

 difficult to measure, but of the order required. 



11. Case of dX/dy, and dQjdy, etc. — If, in equation (12), 

 e and n are constant while /jl, R, y, and X vary, the micro- 

 meter equivalent of the displacement of fringes may be 

 found. Here 



dfi/dy = (d/jL/dX) . [dX/dy) 

 and 



dR/dy = (dU/d^) . (dw'dX) . (dX/dy), 



which coefficients are given by equations (13), (14), and 

 (15). 



Centres correspond to 



tvt t-* eX dii 



N =^COSli— ■ rj -jF- 



n cos K dX 



* American Journal of Science, xxx. 1910, p. 170, 



