424 Lord Rayleigh on the Colours diffusely 



film of collodion deposited on a bright surface of silver 

 shows brilliant colours in reflected light. It, moreover, 

 scatters light of a colour complementary to the colour of 

 the directly reflected light. This is apparently due to the 

 fact that the collodion film 'frills,' the mesh, however, being 

 so small that it can be detected only with the highest powers 

 of the microscope. Commercial ether and collodion should 

 be used. If chemically pure ether obtained by distillation 

 is used, the film does not frill, and no trace of colour is 

 exhibited. Still more remarkable is the fact that if sun- 

 light be thrown down upon the plate at normal incidence, 

 brilliant colours are seen at grazing emergence, if a Nicol 

 prism is held before the eye. These colours change to the 

 complementary tints if the Nicol is rotated through 90°, 

 i. e. in the scattered light, one half of the spectrum is 

 polarized in one plane, and the remainder in a plane perpen- 

 dicular to it." 



I have lately come across an entirely forgotten letter 

 from Rowland in which he describes a similar observation. 

 Writing to me in March 1893, he says : — " While one of 

 my students was working with light reflected from a metal, 

 it occurred to me to try a thin collodion film on the metal. 

 This not only had a remarkable effect on the polarization 

 and the phase but I was astonished to find that it gave 

 remarkably bright colours, both by direct reflexion and by 

 diffused light, the two being complementary to each other. 

 I have not gone into the theory but it looks like the pheno- 

 menon of thick plates as described by Newton in a different 

 form. The curious point is that I cannot get the effect b\ 

 making the film on glass and then pressing it down hard 

 upon speculum metal or mercury although I think tho 

 contact is very good in the case of the speculum metal. 

 Possibly, however, it is not. Gelatine films on metal give 

 good colours by direct reflexion but not by diffused light : 

 only faint ones. It would seem that the collodion film must 

 be of variable density or full of fine particles. However, 

 I leave it to you. I send by express two of the plates used." 

 Probably it was preoccupation with other work (weighing 

 of gases) that prevented my giving attention to the matter 

 at the time. 



Wishing to repeat the observation of the diffusely scattered 

 colours, I made some trials, but at first without success. 

 On application to Prof. Wood, I was kindly supplied with 

 further advice and with a specimen of a suitably coated 

 plate of speculum metal. Acting on this advice, I have 

 since obtained good results, using very dilute collodion 



