Onthe Bands formedby the Superpositionof Paragenic Spectra. 23 



reflexion, the minute serrated bands upon abed become smaller 

 and less serrated. 



When the inclination m n N M of the grooved plates is in- 

 creased, the large bands become smaller and smaller ; and when 

 it is diminished, they become larger and larger, getting inclined as 

 in fig. 3, and becoming parallel at 0° of inclination. 



Having been provided, by the kindness of Sir John Barton, 

 with two grooved plates of glass containing 500 divisions in an 

 inch, I was enabled to examine the fringes on the paragenic 

 spectra under different circumstances. 



When the grooved surfaces of the plates were placed in con- 

 tact, and the grooves formed a small angle with one another, 

 the middle or principal image, A (fig. 4), when observed with a 

 lens whose anterior focus coincided with the grooves, had no 

 bands, but the paragenic spectra a, c, b, d on each side had 

 numerous serrated bands or fringes perpendicular to the direc- 

 tion of the grooves, the number on the first spectra a } b being 

 at the rate of 19 in an inch of the luminous disk, and increasing 

 in arithmetical progression. 



When the luminous object is rectangular, and the rectangular 

 paragenic spectra are brought nearly into contact, as at a b and 

 cd (fig. 5), the bands, as seen, at nearly a perpendicular inci- 

 dence, are shown in this figure. 



When the incident light is inclined to the direction of the 

 grooves, the bands suffer no change, and appear immoveable on 

 the surface of the glass plates. 



When the ray of light is perpendicular to the direction of the 

 grooves, and the surface of the glass on which they are cut is 

 inclined to the ray of light, the bands all descend from a to b 

 (fig. 5), moving off, as it were, at b and d and succeeded by 

 others when the angle of incidence increases, while they ascend 

 from b to a and from d to c, moving off at a and c, when the 

 angle of incidence diminishes. In this case the grooves of the 

 plate next the eye are turned to the left, the opposite motions 

 taking place when they are turned to the right*. 



The bands correspond to the intersection of the one set of 

 grooves with the other set; and consequently they diminish in 

 number and recede from one another when the inclination of 

 the one set of grooves to the other diminishes, becoming parallel 

 to the grooves when the grooves on both plates are parallel; 



Interference bands parallel to the grooves may be seen by 

 transmitted light upon the paragenic spectra when two systems 

 of grooves are placed parallel to each other, and when the 

 grooves in the one system are parallel to those in the other. 



* This motion of the bands is not seen when the grooved surfaces are 

 perfectly parallel. 



