Non-reversed Spectrum Interferometry. 



405 



It is not difficult to obtain the single vertical line (which shifts 

 laterally), black or bright, on suitable rotation about e. On 

 either side of this transitional adjustment the fringes move 

 vertically (climb or fall) in opposite directions, for the same 

 micrometer displacement. The arrow-shaped forms are also 

 often satisfactory, and may be obtained by adjusting the two 

 bright patches on the reflecting grating into coincidence, by 

 the eye, in the absence of the telescope. The grating G' is 



Figs. 4, 5. 



c/H.Jlc JLJlc 



moved fore and aft for this purpose on the slide, s, fig. 2, 

 until the two bright strips become one. 



In making the first adjustment, I incidentally combined the 

 first order spectrum from JV, with the second order spectrum 

 from 3f, as shown in fig. 5, under the impression that the 

 wider D groups from the latter were due to slight curvatures 

 of mirrors. The fringes were nevertheless easily found and 

 showed no anomalies, except that observation had to be made 

 near M or JV. 



It appears from fig. 5 that the equations for this case imply 



sin i + sin Q 

 sin i — sin 6' 



2A/A 



where the angles i and 6 are equal (0/ = 6"). Thus 

 sin i = 3X. / 2Z> 2 and sin 6 = X/ 2D„ Z> 2 being the grating con- 

 stant (Z> 2 = 200 X 10- 6 cm). 



Hence sin i = -4420, sin # 2 = -1423 

 * = 26° W, 2 = 8° 11'. 



while from the first grating, D l — 350 X 10 -6 cm., 



6, = 9°38' 



whence a- 

 8 



i + 

 i — 



= 35° 52' 

 = 16° 36' 



Am. Jour. Sci. — Fourth Series, Vol. XLII, No. 251. — November, 1916. 

 28 



