156 THE INTERFEROMETRY OF 



but it is generally more convenient to use the former (Mg') , reflecting it from 

 the plane opaque mirror m to the telescope at T. When L came from sunlight, 

 or from an arc light, etc., the white images of the slit were very bright. After 

 putting them in coincidence, horizontally and vertically, by aid of the three 

 adjustment screws on the mirror M, Ives prism-grating G may be placed in 

 front of the objective of the telescope. A very brilliant spectrum thus appears, 

 and the fringes are easily found by moving the micrometer slide which carries 

 M to the proper position. In my apparatus gg' was about 50 cm. long and 

 gM = gN about 2 meters. The telescope is sufficiently near M to manipulate 

 the micrometer, the mirror m being so placed that the beam just misses the 

 strip gg'. 



96 



The interference pattern, found at once and satisfactorily centered, consisted 

 of large, broad circles. On moving the micrometer M from evanescence on one 

 side of the center to evanescence on the other, the slide was found to have 

 moved over about 2 mm. With a stronger telescope to magnify the fine, hair- 

 like fringes, this distance would have been larger. It is interesting to compare 

 this datum displacement with the datum found in the case of the phenomenon 

 above, where a range of over 0.5 cm. (double path-difference) was observed. 

 In the present experiment the range is smaller, because the interference pattern 

 falls below the limit of visibility before the possibility of interference is 

 exhausted. Mg' slides along g' when M moves. 



95. A Rowland spectrometer for transmitting and reflecting gratings, plane 

 or concave. In the above experiments I had occasion to examine a variety of 

 gratings, and it was therefore desirable to devise a universal instrument by 

 which this could be accomplished without delay. The method chosen is sim- 

 ilar to that previously described,* but its details have been greatly simplified, 

 on the one hand, and made more generally applicable, on the other. It seems 

 permissible, therefore, to give a brief description. 



* Carnegie Inst. Wash. Pub. No. 149, Chapter I, 1911. 



