REVERSED AND NON-REVERSED SPECTRA. 91 



41. The same. Compensators. A compensator of ordinary plate glass, 

 at the intersection c, figure 57, produces no effect, if symmetrical to both beams. 

 If not symmetrical, the interferences are displaced to right or left in the field 

 of the telescope, as in any case of displacement interferometry, depending on 

 which component beam receives the longer glass-path. Thus this adjustment 

 corresponds to the grating in the preceding paragraph, the difference being 

 that in the latter case the same ruling is used for both diffractions. Hence 

 the interference figures obtained are simpler, showing vertical strands only. 

 In the present case strands occur in all directions. The maxima for oblique 

 positions of the glass plate were not found with reflecting gratings. 



If the compensator is within i inch in thickness, its introduction occasions 

 no difficulty. The interference pattern may be changed, but it remains the 

 same during the rotation of the compensator ; but if the latter is thicker than 

 2 inches, the figure is usually so small as to be found with difficulty, unless 

 the grating G' is brought forward, to allow for the mutually inward refraction 

 of the rays. If this is done, the same figure may be reproduced. On advancing 

 the grating, plate compensators much over 3 inches thick were tested without 

 the slightest annoyance. Lenticular compensators require special adjust- 

 ment and are very difficult of use. 



The effects of rotating the grating about the three cardinal axes have all 

 been considered above. In the present instance two sets of fringes are sym- 

 metrically rotated, subject to the same conditions. Rotation of G' around a 

 horizontal axis requires an elevation or depression of the arc lamp, if the 

 fringes are to remain in the field. Rotation around a vertical axis separates 

 the slit images, and a readjustment for superposition is necessary. Results 

 so obtained are therefore complicated and were not studied. 



42. Miscellaneous experiments. Fringes with mercury light. A few 



random experiments made with the sodium arc, in the presence and absence 

 of the magnetic field, showed no results; nor was this to be expected, as a 

 reasonably strong field would blow out the arc. Again, the insertion of a 

 glass compensator, 0.7 cm. thick, in one of the component beams, developed no 

 maximum on rotating the compensator about a vertical axis. Thus with reflect- 

 ing gratings the peculiar behavior of the transmitting grating, showing a maxi- 

 mum on either side of a symmetrical minimum (36, 37), is not reproduced. 

 The effect of rotating the first reflecting grating G on a vertical axis is only 

 to throw the sodium light out of one side or the other of the (superposed) 

 slit images. No available means of enlarging the fringes indefinitely was 

 found. It is probable that this would require fine adjustment for symmetry. 

 The field of interference, as a whole, is within a spot-like area which may be 

 moved up and down, or right and left, by the vertical and horizontal adjust- 

 ment screws on the mirror M. Coincidence at the two sides of the slit favor 

 different interferences. The case is always as if, at a single point of the field 

 only, there were actual coincidence, and that the interference pattern is 

 grouped closely around it. 



