the Superposition of Paragenic Spectra, fyc. 101 



spectrum being double those on the first. They are seen at all 

 angles of incidence, and are larger and more distinct at small 

 angles. 



When the grating is turned round in its own plane at any angle 

 of incidence, so that the grooves form different angles with the 

 bar of light, the bands cross the spectra and become perpendi- 

 cular to them in the azimuth of 45°. The paragenic spectra 

 are thus divided into a great number of spectra, the number in- 

 creasing as formerly on each succeeding spectrum. 



2. When the grooves are parallel to the bar of light, and the 

 plane of reflexion perpendicular to the grooves, the bands are 

 apparently segments of concentric circles at great angles of 

 incidence, the radius of which increases as the angle of incidence 

 diminishes, so that they become straight lines at a perpendicular 

 incidence. The bands are smaller at their upper and lower, 

 ends, and those on the second spectrum are, as before, double 

 those on the first, as shown in fig. 9. 



In the spectra on the left hand of the bar of light, the con- 

 cave side of the circular bands is towards the bar ; and in the 

 spectra on the right hand of the bar of light the convex side of 

 the circular bands is towards the bar. The bands on the right- 

 hand spectra are smaller and more numerous than those on the 

 left-hand spectra ; and yet, by increasing the angle of incidence, 

 the bands on all the spectra increase in size and diminish in 

 number. 



If at any particular incidence we turn the grating in its own 

 plane, the bands cross the spectra at angles increasing with 

 the degree of rotation, and becoming smaller and more nume- 

 rous. When the end of the grating nearest the eye (A, fig. 8) 

 ascends, the fringes, great and small, diminish and become more 

 distinct, and the centres of the circles descend. When the 

 grating is turned in the opposite direction, the centres of the 

 circles ascend. 



In the principal gratings which I possess, when upon thin 

 glass*, including those of 1000 and 2000 in an inch, these cir- 

 cular banc^s are accompanied by another system of circular bands, 

 convex to the luminous bar when seen on the left-hand spectra, 

 and concave to it when seen on the right-hand spectra; but 

 what is remarkable, they are smaller and more numerous on the 

 first spectrum than on the second, as shown in fig. 10. They 



* These bands are not seen on a beautiful Munich grating, kindly lent 

 me by Professor Stokes, having 3750 divisions in an inch. As the bands 

 become smaller with the thickness of the glass, their absence in this grating 

 arises doubtless from its great thickness, which is 0'158 of an inch, the 

 thickness of the gratings upon which they appear being about 04. 



Phil. Mag. S. 4. Vol. 31. No. 207. Feb. 1866. I 



