162 ELEMENTS OF LABORATORY WORK 



portions of the wave. But now, as the distance from a 

 increases, the consecutive portions of the wave become more 

 nearly equal in area, and at a distance which is very small, 

 on account of the minuteness of the dimensions of light- waves, 

 the areas become practically equal, and joint disturbances from 

 consecutive areas become equal and opposite in their effects. 

 Hence the effective portion of the wave is a small area around 

 the point a in the straight line joining the luminous point 

 with the point of investigation p. Hence an obstacle placed 

 at this point a shuts off the light from P. 



One important result of this explanation is that, if alternate 

 portions, be, de, &c., be stopped by opaque bodies, the total 

 quantity of light reaching p will be increased. This will now 

 be demonstrated by means of an interference grating. 



103. The Interference Grating. In observations with this 

 grating we shall deal with light-waves of one kind that is, of 

 the same length for the sake of simplicity. The grating now 

 used consists of a number of very fine and very close parallel 

 lines, which have been accurately ruled at equal distances by 

 means of a machine on a piece of glass. A photographic 

 reproduction of such a grating answers every purpose. This 

 is now placed in a vertical position on the small central table 

 of the spectroscope, so that the yellow sodium light from the 

 slit of the collimator falls upon it normally. On looking 

 through the grating towards the slit, the direct image of the 

 slit will be seen, and, in addition, several images on each side 

 of the direct image growing fainter and fainter as they 

 increase in distance. On bringing the cross-wires of the 

 telescope to bear upon these images, and carefully adjusting, 

 if needful, the level of any portion of the instrument, their 

 angular distance from the central image, and from each other, 

 may be very accurately read. 



Let a b, a /l b l) a 2 b% represent in section the opaque 

 portions of the grating, p the position of the eye, and o the 

 slit ; then we may consider the grating to coincide with a 

 very large light-wave, alternate portions of which are cut off 

 by the opacity of the lines, while alternate portions are trans- 

 mitted a condition described in the last section. Without 



