430 Mr, P. Cooper on Accidental Colours, 



is well known, that the different shades of dark blue and 

 indigo, including all the colours formed by a mixture of 

 the blue of the spectrum with various additional portions 

 of yiolet, are also called blues ;* when, therefore, red is 

 added to any of these colours, it converts the green, with 

 the proper proportion of violet, to white ; leaving the 

 superfluous portion of violet, if the red has been added in 

 the proper proportion to neutralize the green, free from 

 any other colour.f — If the red be added in too large a 

 proportion, it gives to the violet a tinge of purple, and, 

 if in too small a quantity, it leaves it a little inclined 

 to blue. 



The experiment with which Mr. Tomlinson commences 

 his paper, and which I sliall have occasion to repeat to 

 shew the gradually diminishing sensibility of the eye, 

 furnishes an instance of this want of distinction ; he says, 

 that if we view a sheet of white paper through a disk of 

 green glass, the sudden removal of the disk from the eye 

 will cause the paper to assume a bright red appearance. 

 Now, upon repeating this experiment, I find the accidental 

 colour is pink, which, as it increases in intensity, approaches 

 to crimson. If the glass be a blue green, the accidental 

 colour will be nearer to a red, in the same proportion that 

 the disk is nearer to a blue. 



I make this statement with the less reserve, because, in 

 a subsequent experiment (22) Mr. Tomlinson has himself 

 corrected the error. He says, " I have adopted a very 

 useful mode of testing the true colour of the shadow, by 

 receiving it first upon white paper, and then upon coloured 

 paper; as, for example, boracic acid in alcohol yields a 

 fine light green flame, the shade is pinlt on white ground, 

 violet on blue ground, orange on yellow, &;c." — I could not 



* The accidental colours of these dark blues are various shades of orange, which 

 rapproach the nearer to yellow as the blue approaches to violet. 



t I observe, that some of your pages have been lately occupied in giving a 

 practical account of the art of dyeing ; it is a subject to which I some years since 

 devoted considerable attention, both upon a large scale and experimentally, and I 

 am convinced, that there is no theoretical principle of greater importance to the 

 art, from which it removes many anomalous appearances, than the discovery of 

 simple colours by the neutralization of one of the constituents of compound 

 colours, in the manner mentioned here and in my former paper, (vol. ii. p. 113.) 

 In adding ingredients during the process, to produce an exact shade of colour, a 

 knowledge of this principle is absolutely necessary to insure success. 



