752 Lord Rayleigh on 



observed to be iridescent, but so slightly that the fact might 

 easily escape attention. But when the plate was under 

 water and suitably illuminated, the brilliancy was remark- 

 ably enhanced. (Jpon this question of illumination almost 

 everything depends. The window-shutter of one of the 

 rooms in my laboratory has an aperture about 4 inches square. 

 In front of this the dish of water is placed and at the bottom 

 of the dish a piece of dark-coloured glass. In thp water the 

 plate under observation is tilted, so as to separate the reflexions 

 of the sky as given by the plate and by the glass underneath. 

 In this way a dark background is ensured. At the coiners 

 and edges of the plate the reflected light is white, then follow 

 dark bands, and afterwards the colours which suggest re- 

 flexion from a thin plate. On this view it is necessary to 

 suppose that the iridescent film is thinnest at the outside 

 and thickens towards the interior, and further, that the 

 material constituting the film has an index intermediate 

 between those of the glass and of the water. In this way 

 the general behaviour is readily explained, the fact that the 

 colours are so feeble in air being attributed to the smallness 

 of the optical difference between the film and the glass 

 underneath. In the water there would be a better approach 

 to equality between the reflexions at the outer and inner 

 surfaces of the film. 



From the first I formed the opinion that the films were 

 due to the use of a silicate substratum in the original pre- 

 paration, but as the history of the plates was unknown this 

 conjecture could not be satisfactorily confirmed. No ordinary 

 cleaning or wiping had any effect ; to remove the films 

 recourse must be had to hydrofluoric acid, or to a polishing 

 operation. My friend Prof. T. W. Kichards, after treating 

 one with strong acids and other chemicals, pronounced it to 

 be what chemists would call " very insoluble." The plates 

 first encountered manifested (in the air) a brilliant glassy 

 surface, but afterwards I found others showing in the water 

 nearly or quite as good colours, but in the air presenting a 

 smoky appearance. 



Desirous of obtaining the colours as perfectly as possible, 

 I endeavoured to destroy the reflexion from the back surface 

 of the plate, which would, I supposed, dilute the colours due 

 to the iridescent film. But a coating of black sealing-wax, 

 or marine glue, did not do so much good as had been expected. 

 The most efficient procedure was to grind the back of the 

 plate, as is very easily done with carborundum. The colours 

 seemed now to be as good as such colours can ever be, the black 

 also being well developed. Doubtless the success was due in 



