REVERSED AND NON-REVERSED SPECTRA. 89 



differently situated. It is nevertheless necessary to forestall all misgivings 

 by avoiding glass plates altogether and adapting the methods of figure 57, 

 where reflecting surfaces (front faces) only are present, to the experiment 

 for crossed rays mN and nM. 



In the apparatus as finally perfected, G, figure 57, was a Michelson plate 

 grating and G' a Rowland concave grating, each with about the same grat- 

 ing constant. A strong lens was placed at T for observation at the focus of 

 the concave mirror of G'. The latter was capable of fore-and-aft motion, of 

 rotation about a vertical axis in its own plane and about an axis normal to that 

 plane; G was capable of rotation about a horizontal axis parallel to its plane. 

 Thus the possibility of fore-and-aft motion and the three cardinal rotations 

 for the gratings, together with a micrometric fore-and-aft motion of M, was at 

 hand, as well as the rotation of M and N about horizontal and vertical axes. 



The interferences were found after establishing the coincidence of the yellow 

 homogeneous fields, in the manner described in the preceding paragraph. 

 The fringes were at first small and apparently single, but they could be 

 enlarged at pleasure and the two definite systems separated by fore-and-aft 

 motion of G'. They occupied only a part of the wide yellow slit image, the 

 sodium arc being used. On actuating the micrometer at M there was dis- 

 placement of the interference pattern as a whole, so that the conditions of 

 displacement interferometry are here also implied, though the equations are 

 liable to be different. On rotating M, micrometrically, about a vertical axis, 

 the structure of the interference reticulations changed and was at times 

 reduced to a single set. 



Whenever the arc flashed, or when white light was used, the linear phenom- 

 enon appeared alone, either cross-hatched or longitudinal, depending upon 

 the character of the reticulated pattern for homogeneous light. With sun- 

 light, even after narrowing the blade from the eollimator and screening off 

 red and green light, the phenomenon was faint and hard to find, unless it 

 was produced alternately with sodium arc. 



With the arc freshly charged with sodium, but a single set of interferences 

 or else the linear phenomenon appears, since the broadened sodium lines are 

 equivalent to a continuous spectrum in this region. Not until the excess of 

 sodium has all been evaporated and the sodium lines are normal does the true 

 reticulation show itself. It is interesting to describe two cases of this double- 

 interference pattern, obtained by gradual and successive fore-and-aft motion 

 of the grating G', between limits, while the edges of the two wide-slit images, 

 respectively horizontal and vertical, are kept in contact throughout. 



Suppose the original fine fringes to be nearly vertical ; then the apparently 

 simple fringes, a, figure 66 (their appearance, however, would lead one to sus- 

 pect their simplicity), change to the cord-like strands b, appearing like helices 

 of a very large pitch. Both interference fringes are still nearly parallel, and 

 they cover the whole wide-slit image uniformly. These eventually pass into 

 the square or rectangular reticulation, c, with both systems equally strong. 

 Probably intermediate forms have here been skipped. The system, e, occurs 



