670 Prof. Nihal Karan Sethi on the Diffraction 



2. Some Experimental Results. 



With the liquids moderately warm and the mixture 

 regularly transmitting some part of the spectrum, the two 

 edges of each individual drop are observed to diffract light 

 through considerable ancles and shine with different colours, 



© © ' 



e. g. one edge may be red and the other blue or violet. The 

 whole mixture consequently presents a curious appearance, 

 with some points red and some points blue. On further 

 warming, the region of equality c»f refractive indices is 

 shifted towards the blue end of the spectrum, and the blue 

 edge gradually deepens into a violet colour and ultimately 

 disappears altogether, while the red one becomes more and 

 more yellowish. At a very high temperature, almost that of 

 boiling water, this latter edge becomes very bright and 

 almost perfectly white, with perhaps a yellowish tinge, and 

 this gives to the whole liquid a peculiar sparkling appearance. 



An individual drop of glycerine, however, shows still more 

 interesting effects when suspended at ordinary room temper- 

 ature from a narrow g'ass tube in aflat-sided cell containing 

 some turpentine, When a distant point source of white 

 light is viewed through the drop held close to the eye, the 

 diffracted light shows a number of coloured rings round a 

 more or less white and broad central patch. The colours 

 most prominent in these rings are red and greenish blue, and 

 to some extent yellow. Tl e inner rings are narrower than 

 those in the outer part of the halo, the outermost one being 

 very broad and almost achromatic. The light which forms 

 these rings appears to come from the convex side of the drop, 

 and the eye has to be taken round the edge in order to see 

 the complete rings. The effect could not be observed at 

 higher temperatures, both on account of the drop not 

 remaining easily suspended and the difficulty of keeping the 

 turpentine hot in the parallel-sided cell. 



To fin 1 the explanation of these effects, some observations 

 were made with the somewhat analogous case of a glass 

 cylinder immersed in a parallel-sided cell containing a 

 mixture of carbon disulphide and benzene of which the 

 refractive index could be varied. The cylinder and cell are 

 held at a distance of about half a metre from a narrow slit 

 from which issues monochromatic light of considerable 

 intensity, obtained from a 3000 c. p. quartz-mercury lamp by 

 using a green ray filter. Putting the eye very close to the 

 vessel and viewing the source through the cylinder, one* sees 

 in general a long band of light extending to very large angles 

 on either side and broken up into a number of fringes, the 

 nature of which is entirely different in the two cases in 



