﻿G rutin is with Controlled Groove Form. 



15 



9° but disappeared at 11°. Its disappearance may have 

 been due to its having passed into the blue region, in which 

 the eye cannot see as sharply as in the more luminous 

 regions. In the following table are given the positions 

 of the bands for various angles of incidence. The wave- 

 lengths passing ofT at grazing emergence in the direction C 

 are given in the fourth column. 



Incidence Angle. 



Dark band. 



Bright line. 



Passing off. 



11° 



5500 

 5650 

 5900 

 60.0 

 6100 

 6200 

 6100 

 6560 

 6650 

 6600 

 6720 



5340 

 5650 

 58()0 

 5940 

 6030 

 6200 

 6350 

 6520 

 6600 

 6720 



5150 

 5340 

 5650 

 5800 

 5940 

 6030 

 6200 

 6350 

 6520 

 6600 



9° 



6° 20' . 



4° 55' 



3° oW 



2° 50' 



1° 30' 







1° 10' 



1°50' 



2°40' 





As wiU be seen from this table, the wave-length of the 

 bright line agrees in each ca>e with that oil the light which 

 is passing off the grating in the fourth order. As the 

 grating is turned in a clockwise direction the two bands 

 move towards the region of greater wave-lengths, the bright 

 line a little more rapidly, until at an angle of 1° 50' on the 

 other side of normal incidence the bright line moves into 

 the centre of the dark band, appearing as shown in fig. 1 c. 



This entrance of the bright band into the dark one is very 

 interesting, and should be a valuable clue to the origin of 

 the band-. The wave-length of the dark bands, for in the 

 case of some of the gratings several dark bands appear, 

 bears no relation to that of any of the colours which are 

 passing off the grating at grazing emergence, though in 

 general they seem to correspond to wave-lengths diffracted 

 at angles a little greater than grazing emergence ; in other 

 words, to result from disturbances which come from the 

 steep side of the groove and are reflected against the 

 opposite side. 



It was found that certain anomalies appear when the 

 direction of the electric vector is parallel to the groove. 

 For example, if we hold the nicol in front of the eye and 

 allow unpolarized light to fall upon the grating, the dark 

 band appears in the orange (incid. angle 5°) when the short 

 diagonal of the nicol is horizontal, i.e. when the electric 

 vector is perpendicular to the groove. If now we rotate 



Y2 



