On the Bands formed by Superposition of Paragenic Spectra. 99 



sions, such as 1000 and 1000, the bands are seen upon all the 

 spectra, and sometimes very faintly on the luminous disk ; but 

 when one of the gratings has twice the same number of divisions 

 as the other, such as 2000 and 1000, the bands appear, as already 

 mentioned, only on the 2nd, 4th, 6th, &c. spectra. In such 

 combinations the 1st, 3rd, 5th, 7th, &c. spectra of the 1000 

 grating have no corresponding spectra in the 2000 grating with 

 which they can interfere, whereas, when the divisions in both 

 are the same, all the spectra of the one are superposed upon all 

 the spectra of the other, and therefore bands are produced 

 upon each of them. 



In like manner, if the number of divisions in the one grating 

 is to those on the other as n to \, n being a whole number, the 

 bands will appear only on the spectra n, 2n, 3n, 4ra, &c. 



When a grating of 1000 is placed above one of 2000, I have 

 observed faint bands upon the spectra 1, 3, 5, &c, of the 1000 

 grating, though none of the spectra of the 2000 grating could 

 interfere with them. These bands are more numerous than those 

 between which they lie, and are doubtless produced by the inter- 

 ference of spectra reflected from the plane surfaces of the glass 

 plates with those seen by transmitted light. 



When the gratings of 1000 and 2000 are placed at a small 

 angle, as in Plate II. fig. 1, the grooves being parallel to A M, 

 and the light incident perpendicularly, the bands on the left- 

 hand spectra are parallel and rectilineal, and highly purple and 

 green y as in fig. 2. 



By turning the gratings round A M as an axis, in the direc- 

 tion from D to B, the bands descend from m, as in fig. 3, till 

 they become parallel vertical lines, increasing in number and 

 less coloured, as in fig. 4, the number of bands on the second 

 left-hand spectrum being double those on the first. 



When the rotation is in the opposite direction, from B to D 

 (fig. 1), the bands rise from n (fig. 5) till they become parallel and 

 vertical as before. 



The opposite effects take place when the gratings are placed 

 as in fig. 6, A M and C S being coincident, and when we observe 

 the spectra on the right hand of the luminous disk. The bands 

 now descend and ascend from the same points m, n, now on the 

 outer side of the spectra. 



When the two edges A M, C S of the gratings are not parallel, 

 as they are in fig. 1, but inclined at a small angle, A M S C 

 (fig. 6), then if, when the fringes are parallel at a perpendicular 

 incidence, we turn the gratings round A M as an axis from B to 

 D, the fringes descend from m, becoming smaller and' smaller 

 till they are parallel and vertical ; but when the gratings revolve 

 from D to B, the fringes become larger and larger, less nume- 



