oj Light hi/ Cylinders and Spheres. 671 



which the refractive index /j, of glass is less or greater than 

 //, the refractive index of the liquid which surrounds it. It 

 is hardly necessary to say that the cylinder should be 

 optically good, showing no striae of any kind, otherwise very 

 complicated effects are produced which may entirely obscure 

 the real phenomenon. 



3. Case (a) /n</n'. 



Fig. 1 in the Plate is the photograph of a typical case of 

 the phenomenon observed when the refractive index of the 

 liquid is higher than that of the cylinder. The main feature 

 of this is a number of fringes very narrow at first and 

 gradually increasing in width in directions further and 

 further removed from that of the incident light. (The fringes 

 on one side of the source only are shown in the photograph.) 

 The visibility of the fringes is also not constant. They are 

 almost invisible at small angles, but gradually improve with 

 increasing obliquity of observation till they attain almost 

 perfect visibility and then deteriorate once again. Another 

 important point about these is the fact that they become 

 narrower and more numerous, and extend to greater angles 

 as the difference between //, and /// increases. It is very 

 instructive to perform the experiment with a monochromator 

 in which the wave-length of the light cnn be continuously 

 altered by the mere turn of a screw. As the wave-length 

 for which fj, — /j,'=0 is approached, the fringe system closes 

 up and ultimately disappears altogether. 



A clue to the explanation of these fringes is obtained when 

 the surface of the cylinder is viewed by means of a small 

 magnifying lens. It becomes clear at once that the whole 

 of the light which forms this system of fringes arises from 

 near one or the other of the edges of the cylinder. At each 

 of these edges, one can clearly distinguish two very sharp 

 and bright luminous lines on the surface of the cylinder 

 from which the light appears to emerge, and which evidently 

 act as interfering sources giving us the observed system of 

 fringes. These luminous lines appear to be well separated 

 from each other when observed at a small angle to the 

 direction of the incident rays, and one of them is very much 

 brighter than the other. As the direction of observation 

 becomes more and more oblique, not only does the distance 

 between the lines become smaller and smaller until it vanishes, 

 but the brighter one diminishes and the feebler one increases 

 in intensity until they become equal at a certain stage, 

 beyond which the latter becomes the brighter o( the two. 



