108 THE INTERFEROMETRY OF 



Figure 77 shows that the entering undivided beam LG passes just 

 above the mirror M, and is reflected just below this from the top of N. 

 Similarly, the reunited beam GT passes just above M, but is reflected from 

 the top of M , the object being to make the vertical angle at G as small 

 as possible. 



The mirrors M and N and the grating G are on adjustable bases, a, a', a", 

 each controlled by three leveling screws on a plane-dot-slot arrangement in 

 the tablets m, m f , m", the axis of rotation being horizontal and normal to the 

 diagram. The tablets, furthermore, may be revolved and raised or lowered 

 by the rods n, n', n", which are attached by ordinary clamps to the large, 

 tubular, horizontal rail, RR, in question, admitting of a circuit of water. The 

 latter is secured to the pier. 



The angles of inclination of the figure are much exaggerated, since the dis- 

 tance MG = GN (nearly) is from one-half to several meters in extent. 



The mirror M is on a Fraunhofer micrometer suggested at m. The bases, 

 a, a', a", are drawn to the tablets, m, m', m", by firm springs, preferably run- 

 ning into the tubes below them. 



The axis of the adjustment telescope, T, lies in the plane of the figure and 

 serves the purpose of bringing the direct slit images into horizontal and vertical 

 coincidence. When this is done it may be removed, if desirable, as the ray 

 GT is not thereafter used. T should not be attached to the rail, but placed 

 on an independent table, or standard, so as not to be an integrant part of the 

 interferometer. The telescope, T (not shown), for the observation of the 

 interferences, should be independently mounted on the same table. This tele- 

 scope lies outside of the diagram, to the right or the left of it, to catch either 

 of the two diffraction spectra selected. It will be seen that these lie quite 

 above the direct diffraction spectra of the ray LGM. Otherwise, as this is 

 much more intense, it would completely wipe out the interference spectra 

 and their combination. The latter, when seen alone, are very brilliant, black 

 and colored patterns, running through the spectrum when the micrometer, 

 m, is manipulated. If the distance GN is large and the grating G, as usual, 

 slightly wedge-shaped, the superfluous rear reflection from G may be blotted 

 out at N by a small screen. It is easily recognized, as it is brown from 

 scattered light. 



The installation is simple. The parts being adjusted nearly symmetrically, 

 the undivided ray from a wide slit is brought to the top of M by raising or 

 lowering the lamp. This should first be done roughly with the lens and slit 

 removed. N has at the same time been placed just below the beam, and this 

 passes through the middle part of G. The latter is then inclined by the adjust- 

 ment screws until the component beam GN strikes the top of N, symmetri- 

 cally. Next N is inclined and rotated (vertical axis) until the reflected beam 

 enters the telescope, T. Finally, M is inclined and rotated (vertical axis) 

 until the reflected rays MG and GT also enter the telescope, the final sharp 

 adjustment being made with a narrow slit and the eye at the telescope. 

 The mirror M must also have a fine vertical adjustment (not shown). If the 



