676 Diffraction of Light by Cylinders and Spheres. 



observation gradually made more and more oblique. At 

 the edge o£ the cylinder appeared a sharp brilliant line 

 at a certain angle of observation, and this gradually changed 

 its colour from violet to blue, green, yellow, and then red. 

 After a short interval the colours were repeated, showing 

 a second-order caustic {{jl was less than /x throughout the 

 visible spectrum and the difference fi—fi least for violet 

 rays). These colours are identical with rods of different 

 radii, and even the angles at which they are observed are 

 the snrne with such widely different diameters as 8 mm. 

 and *5 mm. But these angles get smaller and smaller 

 when fjj is increased by the addition of more carbon di- 

 sulphide. 



Though it is difficult to make any definite assertion at the 

 |. resent stage, it appears probable that the sparkling of the 

 drops in the mixture of glycerine and turpentine is due to 

 the formation of these caustics, the absence of colour being- 

 due to the relatively much smaller relative dispersion of the 

 liquids. 



6. Summary. 



When equal quantities of glycerine and turpentine are 

 shaken up together, we observe that in addition to the usual 

 Christiansen phenomenon shown by the mixture, the indi- 

 vidual drops in it exhibit some interference and diffraction 

 effects, and at ordinary temperatures their edges appear 

 variously coloured. A source of light observed through a 

 single drop of glycerine suspended in turpentine appears 

 surrounded by a number of coloured rings. These cases are 

 similar to that of a glass cylinder immersed in a mixture of 

 carbon disulphide and benzene whose refractive index is 

 slightly greater. The fringes obtained in this case with 

 monochromatic light are shown in PI. XX. tig. 1. Their 

 width increases with the value of the angular deviation of 

 light forming them, and their visibility, which is very poor 

 at small angles, considerably improves at greater obliquities, 

 but once again it falls off towards the end where the fringe- 

 system terminates in a very broad band. With glass spheres 

 these fringes become circular rings with the same character- 

 istics. It has been shown that this phenomenon is due to 

 the interference of rays totally reHected from the convex 

 surface of the cylinder or the sphere and those twice 

 refracted by them. The distribution of light in the pattern 

 has been calculated in a typical case, and it is found to 

 accord with the experimental results both with regard to the 

 width as well as the visibility of the fringes. 



