490 C. Barus — Interferences of Grossed Spectra. 



the lines may become longer or all but coalesce and their incli- 

 nation may change somewhat. They nevertheless remain fine 

 and nearly vertical until they vanish completely and there is 

 no rotation. Nor could the phenomenon be found again 

 within the length of the given micrometer screw. Hence it is 

 improbable that these interferences conform at once to the 

 ordinary elliptic type, even if the ellipse is considered excep- 

 tionally eccentric. The use of two slits, one following the 

 other, does not change the pattern. 



The modified method of experiment was one of double diffrac- 

 tion. In figure lb, L is the blade of light from the collimator, 

 which passing under the plane mirror m, penetrates the grat- 

 ing G whence the diffracted first order beams reach the opaque 

 mirrors Jf and JV. These return the beams nearly normally, 

 but with an upward slant, so that the color selected intersects 

 the grating at a higher level than L. A second diffraction 

 takes place at the same angle, 0, 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 an easy adjustment, as everything is controlled by the 

 adjustment screws on M and N and plane mirrors M, JY, and 

 m only are needed, the latter being on a horizontal axis to 

 accommodate T. Directly transmitted white light is screened 

 off. 



3. The same. Further experiments. — In place of the plane 

 mirror, m, a slightly concave mirror (two meters in focal dis- 

 tance, 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. 1, b, 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 measure- 

 ably revolvable about a vertical axis. In this case any unknown 

 wave length, X', is obtained by rotating M, until X' is in coin- 

 cidence with X. Supposing the X's of the two spectra to have 

 been originally in coincidence and that 9 is the angle of 21 

 which now puts X' in coincidence with X, it is easily seen that 



X- A = A (2sin 2 / 2 + V D % / A a - lsin0). 



Angles must in such a case be accurately measureable, i. e. to 

 about "1 minute of arc per Angstrom unit, if D = 351 X 10"°, 

 as above. Counter rotation of the mirror JV till the X"s 

 coincide would double the accuracy. The usual grating, 

 however, has greater dispersion and would require less preci- 

 sion in 6. 



Finally a still simpler and probably more efficient device 

 consists in combining the mirror m and the plane grating G> 



