400 Prof. R. W. Wood on Uneven Distribution of 



angle of 5° 45', This band is much wider than the others and 

 seems to be absolutely black at the Centre, even with a 

 fairly wide slit. 



This represents the cycle when the grating is in air. If 

 a piece of plane-parallel glass is cemented to the front of the 

 grating with cedar oil the cycle is quite different. In this 

 case we have a pair of unsymmetrical shaded bands which 

 move in the same direction as the angle of incidence is 

 changed. In fig. 2 I have given the appearance and position 



Fig. 2. 



69 68 67 66 65 64- 63 62 61 60 59 58 57 56 55 54 53 51 51 50 49 48 4-7 46 4-5 

 J 1 I I I I I I I I L 1 L 



Incidence on 

 same side of 

 Normal as < 

 Spectrum. 



Incidence on 

 other side z°zs' 



of Normal. 



I 



I 



I 



of these bands for three different angles of incidence. It 

 will be observed that they remain distinct on passing through 

 the position of perpendicular incidence. 



It is impossible to identify these bands with those observed 

 with the grating in air, since the jump in the refractive index 

 of the medium in which the grating is immersed is too great. 

 To determine the effect of increasing the refractive index 

 of the medium on the position of a given band I fastened a 

 plate of glass in front of the grating at a distance of about 

 0'3 mm. from the ruled surface. Water was introduced 

 between the two, and glycerine applied to the lower edge. 

 The denser fluid gradually diffused up into the water, and 1 

 observed the dark bands sharply curved, on looking at the 

 spectrum directly in the grating without the aid of a telescope, 

 the shift being towards the red, as the refractive index 

 increased. The appearance of one of the bands is 

 in fig. 3. It will be observed 

 that the shift is in the same 

 direction as when a resonator is 

 immersed in a medium of high 

 dielectric constant, and though 

 there may be no connexion be- 

 tween the two phenomena, it 

 seems perhaps worth while to 

 mention it as there may be some- 

 thing akin to resonance in the action of this grating. 



diown 



