104 



THE INTERFEROMETRY OF 



ences are visible is not above 2 mm. of displacement of M. The fringes may, 

 as usual, be made as large as possible, by first superposing the two illuminated 

 strips on the grating G (by fore-and-aft motion) and then rotating the grating 

 on an axis normal to its face until the best conditions appear. Both spectra 

 are very bright, but liable to be in different focal planes from inadequate 

 planeness of the reflecting system. If work of precision is aimed at, this con- 

 dition is of foremost importance. 



54. Another method. If the opportunity of using crossed pencils of white 

 light is to be dispensed with, the prism method may be simplified, as shown 

 in figure 74. Here P is a prism with silvered sides and a prism angle of less 

 than 30. It receives horizontal white rays L from a collimator, which, after 

 reflection from the opaque mir- 

 rors M and N, impinge on the 

 grating G, plane or concave, and 

 are observed at T by a telescope 

 or lens. 



If <p is the prism angle and 6 

 the angle of diffraction, it is 

 easily seen that the angle be- 

 tween the rays reflected at M 

 or N is 



d=6-<p 



Hence, if P is a 30 prism, the 



observations can be made only 



in the second-order spectra. If cJlf 



observations in the first order 



are desired because of the greater illumination, <^ must be less than 20, as a 



rule, for a grating of about 15,000 lines to the inch. The mirrors M and N 



make an angle of a/ 2 = (<p-\-d)/2 with the line MN. 



The first experiments were made with a 30 prism and second-order spectra 

 from a concave grating (.0=177X10-* cm.). Sunlight was used. The two 

 superposed spectra were magnificent, with abundance of light and high disper- 

 sion ; but the spectra were of unequal intensity and in different focal planes, 

 so much so that the images of the guiding horizontal thread of the spectra 

 could scarcely be seen together. This made the adjustment for coincident 

 longitudinal axes very difficult, and the interferences were not found until after 

 long trial. The reason for this is the probable concavity or convexity of one 

 or more of the reflecting surfaces. Another difficulty was the distance apart 

 of the mirrors M and N (roughly, 150 cm. for a distance of about 2 meters 

 from P to T), so that it was inconvenient to observe and actuate the mirror 

 micrometer at M. Further attempt at improvement was therefore abandoned. 



This prism was now replaced by one of less angle than <p = 2o, also well 

 silvered. In the first experiments the adjustment did not admit of a coinci- 

 dence of light, except near the C line of the red; but M andN" were now less 

 than oo cm. apart, while the distance between G and T was about no cm., and 



