102 Sir David Brewster on the Bands formed by 



are best seen when the principal circular bands cross the spectra 

 obliquely. 



In the preceding experiments with one grating the grooves of 

 the reflected image are necessarily parallel to those of the real 

 grating, owing to the parallelism of the surfaces of the plate 

 of glass, and therefore they cannot exhibit the result of super- 

 posing two systems of grooves inclined to each other. This 

 condition, however, may be obtained by drawing the grooves 

 on the faces of a prism with a small angle, or by placing a 

 fluid prism between an ordinary grating and a plate of thin 

 parallel glass, which would enable us to vary the inclination 

 of the two sets of grooves. A better arrangement, however, 

 is to place the grating AB (fig. 11) upon a polished metallic 

 surface, M N. A ray from the luminous bar at R, incident 

 on A C at r, reaches the eye at E after reflexion from the steel 

 surface M N, so that the reflected image of the grating A B 

 is superposed as it were on the direct image. 



When the grating AB, of 1000 grooves in an inch, is laid 

 upon a steel surface M N, and the grooves are in the plane of 

 incidence, the paragenic spectra of a luminous bar are covered 

 with bands, not serrated, parallel to the spectra, exhibiting all 

 the phenomena already described as seen by reflexion from a 

 single grating. 



The bands are of the same size as with a single grating when 

 the grooved surface is uppermost, but they are very much larger 

 when the grooved surface is in contact with the steeL 



When the grooved surface is slightly inclined to the steel sur- 

 face, as in fig. 11, and the grooves parallel to the plane of re- 

 flexion, a double system of hyperbolic bands is seen, as in 

 Plate III. fig. 12, having one asymptote coincident with the 

 bar of light and the other at right angles to it. One of the sys- 

 tems of hyperbolas is on one side of the bar and the other sys- 

 tem on the other side, the number of bands on the second spec- 

 trum being double those on the first. 



When the grooves are inclined to the plane of reflexion, by 

 turning them to the left or to the right, the double system of 

 hyperbolas moves to the left or to the right, the curves of each 

 system crossing the spectra, as in fig. 13, and being, as before, 

 twice as numerous on the second as on the first spectrum on 

 both sides of the bar. By increasing the inclination of the 

 grooves to the plane of incidence, the system of hyperbolas 

 moves further to the left or to the right. 



When the bar of light is placed at E and the eye at B, (fig. 11), 

 the system of hyperbolas is inverted, as in fig. 14. 



It is curious to observe the passage of the parallel rectilineal 

 bands into hyperbolas, when the inclination of the grooved to 



