624 



Mr. C. V. Raman on the Pliotometiic 



Lands on one side of the pattern lie on the brighter annuius 

 the narrow ones on the other side in the feebler annuius. 



The general disposition of the apparatus when at work is 

 shown in the diagram below. The light from the slit S 





a^KSH' 



a-WVW- 



cl^/V\AA. 



J 



passes directly and partly also after reflexion at the surface 

 of the prism P to the lens of a camera. The image of the 

 slit and the diffraction-pattern are focussed by the lens on to 

 the plane of the photometric disk, the adjustments being as 

 described above. DD is a screen placed in front of the 

 prism to shut off" superfluous light. To further reduce the 

 amount of scattered light, nil the faces of the prism except 

 the one used are completely blackened with a coat of varnish. 

 The pattern is observed with the aid of a lens L placed 

 behind the photometric disk. The central bright band of the 

 pattern which would otherwise dazzle and confuse the eye, 

 is cut off by a vertical wire W placed in front of the eyepiece. 



The relative intensity of the first bands on either side of 

 the central band is observed. Their brightnesses are adjusted 

 to equality by sliding the disks one over the other, and 

 thereby altering the angle of the smaller sectors. To secure 

 accuracy, it was found essential to have the bands well- 

 illuminated by using a brilliant source (in practice sunlight) 

 to light up the slit. The adjustment to equality of bright- 

 ness was found to be greatly facilitated if the upper or lower 

 edge of the bands stood out sharply against a perfectly dark 

 background. 



When the adjustment is made, the lengths of the chords of 

 the six larger and the six smaller sectors are read off with a 

 scale. A divided glass scale is then placed in the focal plane 

 of the camera so that the diffraction-pattern falls upon it, 

 and the positions of (a) the direct image of the slit, (b) the 

 maximum intensity in the central band, (c) and (d) the 

 maximum intensities in the bands compared, are read off and 

 noted. The theoretical ratio of the illuminations can be com- 

 puted from these measurements and compared with the value 



