﻿434 Lord Rayleigb on Departures from 



was mounted in a bottle prism in such a manner that the 

 incidence could be rendered grazing. When the adjustment 

 o£ indices was for the yellow, the appearances observed were 

 as follows : i£ the incidence is pretty oblique, the reflexion 

 is total for the violet and blue ; scanty, but not evanescent, 

 for the 3-ellow ; more copious again in the red. As the 

 incidence becomes more and more nearly grazing, the region 

 of total reflexion advances from the blue end closer and closer 

 upon the ray of equal index, and ultimately there is a very 

 sharp transition between this region and the band which now 

 looks very dark. On the other side the reflexion revives, 

 but more gradually, and becomes very copious in the orange 

 and red. On this side the reflexion is not technically total. 

 If the prism be now turned so that the angle of incidence is 

 moderate, it is found that, in spite of the equality of index 

 for the most luminous part of the spectrum, there is a pretty 

 strong reflexion of a candle-flame, and apparently without 

 colour. With the aid of sunlight it was proved that in the 

 reflexion at moderate incidences there was no marked 

 chromatic selection, and in all probability the blackness of the 

 band in the yellow at grazing incidences is a matter of 

 contrast only. Indeed, calculation shows that according to 

 Fresnel's formula?, the reflexion would be nearly insensible 

 for all parts of the spectrum when the index is adjusted for 

 the yellow." It was further shown that the reflexion could 

 be reduced, but not destroyed, by re-polishing or treatment 

 of the surface with hydrofluoric acid. 



I have lately thought it desirable to return to these experi- 

 ments under the impression that formerly I may not have 

 been sufficiently alive to the irregular behaviour of glass 

 surfaces which are in contact with the atmosphere. 1 wished 

 also to be able to observe the transmitted as well as the 

 reflected light. A cell was prepared from a tin-plate cylinder 

 3 inches long and 2 inches in diameter by closing the ends 

 with glass plates cemented on with glue and treacle. Within 

 was the glass plate to be experimented on, of similar dimensions 

 as to be nearly a fit. A hole in the cylindrical w^all allowed 

 the liquid to be poured in and out. Although the plate 

 looked good and had been well wiped, I was unable to re- 

 produce the old effects ; or, for a time, even to satisfy myself 

 that I could attain the right composition of the liquid. 

 Afterwards a clue was found in the spectra formed by the 

 edges of the plate (acting as prisms) when the cell was slewed 

 round. The subject of observation was a candle placed at a 

 moderate distance. When the adjustment of indices is 

 correct for any ray, the corresponding part of the spectrum 



