232 BANDS FORMED BY THE SUPERPOSITION OF PARAGENIC SPECTRA, ETC. 



second spectrum being double those on the first, as before, and becoming narrower 

 and closer as they recede from the centre. When the first and second spectra are 

 close to one another, as in fig. 17, the rings upon entering the second spectrum 

 are doubled, as shown at mmm. These rings are seen only when the grooves 

 are inclined to the plane of reflexion. By increasing the inclination, they be- 

 come smaller and more distinct, their size being a minimum, and their distinctness 

 a maximum, when the azimuth of the grooves is 90°. When the azimuth is 0°, 

 or when the grooves are turned into the plane of reflexion, the rings open, as 

 at fig. 18, and when turned into azimuth 1° or 2°, those on the side ab, fig. 18, go 

 back to the left, and those on the side cd bend into a ring, as shown in fig. 19. 

 When the rings are again formed, they increase as the angle of incidence 

 diminishes. 



When the rings are increasing or diminishing, or passing from one spectrum 

 to another, their centres are sometimes white, and at other times so black as to 

 eclipse the rings of Newton. Their colour is very variable, sometimes black, with 

 colourless intervals, and sometimes richly coloured with the tints of the spectra 

 on which they are seen. When the grating is pressed upon the convex surface, 

 or raised slightly from it, the rings exhibit the same phenomena as those of thin 

 plates. 



When the ray RR' (fig. 19) from the bar of light, reaches the eye at E, the 

 grooves being slightly inclined to the plane of reflexion, the hyperbolic bands 

 are seen, as in fig. 12, and when the ray ?V reaches the eye at e, the hyperbolic 

 bands are seen as shown in fig. 13, and when the eye receives all the rays be- 

 tween R' and r\ the direct and inverted systems of hyperbolas are seen, as in fig. 

 20. If, when these are seen, we look at the surface of the grating, we shall see 

 the system of concentric rings produced by the union of the two systems of 

 hyperbolas. 



