620 Mr. C. V. Raman on the Plwtomet 



ric 



One method o£ verifying the theory suggested would 

 probably be to show that no other explanation of the observed 

 effects is feasible. Any suggestion about defects in the 

 experimental arrangements was found to be inadmissible. 

 The focussing of the bands was or could be rendered perfect. 

 The reflecting surface used was the face of a first-class prism 

 of the kind used for accurate spectrometric work, and was 

 absolutely fresh and clean : in fact, no perceptible effect was 

 found to be produced by any moderate amount of dust or 

 grease, the reason apparently being that the incidence was 

 very oblique. The effect of scattered light, if any, in the 

 field was in the opposite direction, and it could almost 

 entirely be excluded by special precautions (to be described 

 below in connexion with the photometric work). In fact, 

 all these factors were found to be quite incapable of 

 accounting for the observed effects. 



Even assuming that there was at work in the experiments 

 some unknown factor (other than an obliquity-factor), it is 

 difficult to see how it could have altered the relative intensity 

 of the bands on the two sides of the pattern to a considerable 

 degree without either (1) shifting the positions of the minima 

 of illumination from those given by the ordinary theory of 

 diffraction ; or (2) without causing a finite sensible intensity 

 to exist at the minima, which according to the ordinary 

 theory are absolute zeroes of illumination. Both of these 

 points were carefully tested by direct observation and by 

 photography. With a sufficiently homogeneous but powerful 

 source of light, there is no difficulty in observing perfectly 

 black bands alternating with bright bands of great intensity. 



© © © 



To secure a satisfactory negative showing clear minima, it is 

 necessary first carefully to abolish halation by backing the 

 photographic plates before use. If care is taken to avoid 

 chemical fogging of the plates, it is possible to get photo- 

 graphs of the diffraction-pattern showing great density at 

 the maxima with practically clear glass at the minima. 

 An enlargement from such a negative is shown as fig. 1 

 in Plate VII. In the diffraction-pattern photographed the 

 theoretical ratios of illumination, calculated by a method to 

 be explained below, should have been as follows : — 



The 1st narrow bright band 1*81 times as intense as the 1st broad band. 

 2nd „ „ 2-92 „ „ 2nd ,, „ 



3rd „ „ 5-68 „ „ 3rd „ 



4th ., „ about 20 „ „ 4th „ „ 



which is too faint to be 

 visible in the negative. 



