222 SIR DAVID BREWSTER ON THE BANDS FORMED BY THE 



When the inclination mriNM. of the grooved plates is increased, 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 contact, 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 direction of the grooves, 

 the number on the first spectra a, b, being at the rate of 1 9 in an inch of the 

 luminous disc, and increasing in arithmetical progression. 



When the luminous object is rectangular, and the rectangular paragenic 

 spectra are brought nearly into contact, as at ab and cd (fig. 5), the bands, as 

 seen at nearly a perpendicular incidence, 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 inci- 

 dence 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. They are seen both at a perpendicular incidence and when the plates are 

 inclined in a plane parallel to the grooves. 



These bands become narrow as the distance of the two grooved surfaces is in- 

 creased, and they are seen at all angles of incidence, and in all planes of reflexion 

 from the grooved surfaces. 



I have observed those bands, which are generally more or less serrated, in com- 



* This motion of the hands is not seen when the grooved surfaces are perfectly parallel. 



