124 



THE INTERFEROMETRY OF 



glass wedge, e e' thick at the ray in question. The possibility of throwing 

 these fringes into any order of size, their small extent, sharpness, and great 

 abundance of light constitute their value for measurement. 



Thus it is furthermore obvious that the achromatic fringes must also be 

 obtainable in Michelson's interferometer, in any order of size and in a field of 

 white light. Tests were made with this object, beginning with the fine slit 

 and the centered ellipses of the spectrum interferences. Removing the spec- 

 troscope and enlarging the slit indefinitely, the residual fringes appeared. 

 They were never so strong and clear, however, in any order, as was the case 

 with the Jamin interferometer, although many trials with different compen- 

 sators were made. They would not be useful for measurement. Removing 

 white light and replacing it by sodium light, the white fringes were found to 

 be of exceedingly high order, more than 0.7 cm. of micrometer-screw being 

 needed before the circles of the yellow field were approached. The latter 

 were very vague and usually not seen in the principal focal plane of the tele- 

 scope. Since the rays retrace their path in the Michelson interferometer, the 

 raising and lowering of the spectrum ellipses is not possible; but the residual 

 fringes may be put in any order by changing the differential glass-path of the 

 rays. They appear but once, not rhythmically like the sodium fringes. 



The equation for this phenomenon would thus be (since the rays retrace 

 their paths) 



n\ = 2eiJ, cos (r a) 



where e is the thickness of the single half -silver and a its effective wedge angle, 

 positive or negative. 



85 



84 



To obtain the circular fringes in Michelson's interferometer with a wide 

 slit and homogeneous light, the rays must rigorously retrace their path 

 i.e., all reflection must take place at the same spot on the half -silver plate. 

 When this is the case the fringes, even though obtained with common plate- 

 glass and a sodium flame, are beautifully circular and sharp. This is due to 

 the fact that so small a part of the plate is used. They are stationary and 

 exhibit the Fizean periods due to the doublet DJDi, admirably. With white 

 light the fringes are faint and useless. When the rays do not accurately 

 retrace their paths i.e., when there are two spots of light on the half -silver 

 one or more centimeters apart the fringes are soon linear and very small, 

 as above. 



With regard to the last equation, if N is the difference of normal distances 

 to the two opaque mirrors (M, N) of the Michelson interferometer, from the 



