on Coloured Shadows. 53 



same place, the same varieties in the strength of the 

 tints of the coloured shadows may be produced merely 

 by opening the window-shutter a little more or less, 

 and rendering the illumination of the paper by the 

 light from without stronger or weaker. By either of 

 these means the coloured shadows may be made to 

 pass through all the gradations of shade, from the 

 deepest to the lightest, and vice versa ; and it is not a 

 little amusing to see shadows thus glowing with all 

 the brilliancy of the purest and most intense prismatic 

 colours, then passing suddenly through all the varieties 

 of shade, — preserving in all the most perfect purity of 

 tint, — growing stronger and fainter, and vanishing 

 and returning at command. 



With respect to the causes of the colours of these 

 shadows, there is no doubt but they arise from the 

 different qualities of the light by which they are illumi- 

 nated ; but how they are produced does not appear 

 to me so evident * That the shadow corresponding to 



* I ought to inform the reader that when the above was written I had not 

 the smallest recollection of what, many years before, I had read concerning 

 coloured shadows, in Priestley's History of Optics. It may perhaps be thought 

 (by others, as well as by myself) that it was a fortunate circumstance that I 

 had forgotten what I had read ; for it left my mind in perfect freedom to pur- 

 sue, in my own way, the investigation of the causes of the phenomena which 

 presented themselves to my observation, without my being biassed by the opin- 

 ions of others, who, before me, had attempted to explain them. Had I recol- 

 lected what others had done, I should not, most probably, have given myself 

 the trouble of engaging in the prosecution of these inquiries. 



But although at the time when this paper was wi'itten I had really no remem- 

 brance whatever of what had been written and published before on this subject, 

 yet soon after the paper was finished, and some time before it was sent to 

 England to be laid before the Royal Society, I was, by an accidental circum- 

 stance, made to recollect what I had so entirely forgotten. Shall I confess 

 what the motives were which induced me to expose myself to the danger of 

 being thought ignorant, or something worse, by suffering my paper to go out of 

 my hands without alteration ? When the glow of the sudden blush which I 

 felt on discovering my danger had passed off, and I had taken time to reflect 

 coolly on all the circumstances of the case, I concluded that it might be useful 



