the Superposition of Paragenic Spectra, fyc. 103 



the steel surface commences : the parallel bands open at their 

 lower end, as in fig. 13, or at their upper end, as in tig. 14, and 

 change into hyperbolas. When the light was strong, I observed 

 a second but fainter system of hyperbolas lying between the 

 principal system and the luminous bar, and caused probably by 

 reflexion from the second surface of the grating. The effect pro- 

 duced by the crossing of the bands arising from these two sys- 

 tems of hyperbolas was remarkable, and similar to what I had 

 observed in combining two gratings of 500 divisions in an inch. 

 This second system of hyperbolas was most distinct when the 

 plane of reflexion from the surface of the steel was coincident 

 with the plane of reflexion from the glass ; and the double system 

 was seen with grooved surfaces of 500, 1000, and 2000 divisions 

 in an inch. 



In using accidentally a steel surface that was not perfectly flat, 

 I was surprised to observe that the bands were not hyperbolas, 

 but circular rings varying in form and size with the angle which 

 the grooves formed with the plane of reflexion. In order to 

 examine this new and beautiful phenomenon, I placed the grooved 

 surface of the grating AB upon a convex surface of steel, MN, 

 as in fig. 15, so that the rays from the luminous body might 

 reach the eye at E, after reflexion from the convex surface MN. 

 The reflected image of the grating is thus superposed upon the 

 direct image, and two systems of concentric rings are seen upon 

 the surface of the grating. At the point of contact, C, and 

 around it, are seen the rings of thin plates described by Newton, 

 and increasing in size with the radius of the surface M N. 

 Around and concentric with these, as shown at a b, fig. 16, is 

 seen a beautiful system of serrated rings formed upon the 

 paragenic spectra, the number of rings upon the second 

 spectrum being double those on the first, as before, and be- 

 coming narrower and closer as they recede from the centre. 

 When the first and second spectra are close to one another, the 

 rings upon entering the second spectrum are doubled, as shown 

 at mmm, fig. 17. These rings are seen only when the grooves 

 are inclined to the plane of reflexion. By increasing the inclina- 

 tion they become 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 



12 



