REVERSED AND NON-REVERSED SPECTRA. 11 



grating at a higher level than L. A second diffraction takes place at about 

 the same angle, 6, to the direct ray t, and the coincident rays now impinge 

 on the mirror m. They are thence reflected into the telescope at T. This 

 method admits of easier adjustment, as everything is controlled by the adjust- 

 ment screws on M and N. Plane mirrors M, N, and m only are needed, the 

 latter being on a horizontal axis to accommodate T. The direct (white) 

 beam is screened off after transmission through the grating, if necessary. 

 But it rarely enters the telescope. 



3. The same. Further experiments. In place of the plane mirror, m, a 

 slightly concave mirror (2 meters in focal distance, say) may be used with 

 advantage and the telescope T replaced by a strong eyepiece. In this way 

 I obtained the best results. 



It is to be noticed that the apparatus (fig. 3) may serve as a spectrometer, 

 provided the wave-length X of one line and the grating space D are known, 

 and the mirror, M, is measurably revolvable about a vertical axis. In this 

 case any unknown wave-length, X', is obtained by rotating M until X' is in 

 coincidence with X. Supposing the X's ofthe two spectra to have been origi- 

 nally in coincidence and that 6 is the angle of M which now puts X' in coin- 

 cidence with X, it is easily shown that 



X'-X = X (2 sin 2 0/2 + \ 2 A 2 -isin 6) 



Angles must in such a case be accurately measurable, i.e., to about o.i minute 

 of arc per Angstrom unit, if the grating space jD = 35iXicr 6 , as above. 

 Counter-rotation of the mirror N till the X's coincide would double the accu- 

 racy. The usual grating, however, has greater dispersion and would require 

 less precision in 9. 



Finally, a still simpler and probably more efficient device consists in com- 

 bining the mirror m and the plane grating G, or of proceeding, in other words, 

 on the plan of Rowland's method for concave reflecting gratings. In such 

 a case the light would enter in the direction TG, figure 3, be reflected along 

 GM, back along MG, and then return along GT at a slightly higher or lower 

 level than on entering. The equation just given would still apply, and many 

 interesting modifications are suggested. Experiments of this kind are to be 

 tested. Moreover, in case of the plane-transmitting grating and plane 

 mirror, as above shown, the same simplification is possible if the lens is 

 replaced by the telescope at T. But in this case the spectra are intersected 

 by strong, stationary interferences due to reflections from front and rear 

 faces and consequently not conveniently available. A reflecting grating 

 and telescope would not encounter this annoyance. In general, however, 

 as in the disposition adopted in figure 3 , the light enters opposite the observer, 

 and, as the light directly transmitted can be screened off, this is a practical 

 convenience in favor of the transparent grating. The reflected spectra used 

 may be placed at any level by rotating the mirror m on a horizontal axis. 



On further repeating the work by the use of the concave mirror m, a strong 

 eyepiece at T, figure 3, and using a compensator, I eventually succeeded in 



