432 Mr, P. Cooper on Accidental Colours, 



intended for the purpose, must be conducive to the same 

 important object. If we can prove the existence of the 

 arrangement, we may safely rely for its utility upon the 

 wisdom and benevolence which made it part of the general 

 scheme. I admit, however, that as philosophers it is our 

 duty to trace the arrangement to its object. 



Mr. Tomlinson very properly objects to the want of com- 

 prehensiveness in the different theories which have been 

 advanced to account for accidental colours ; (27) and I fully 

 agree with him, that no theory is entitled to our confidence 

 that does not embrace the whole of the phenomena within 

 its circuit ; but there may be a question, whether the fault 

 is not in the application of the theory, rather than in the 

 theory itself; and this is the question which I shall now 

 endeavour to decide. 



If the prevailing colour of light disappears in the manner 

 we have been led to infer from the preceding experiment, 

 and which will be still more apparent, if, instead of looking 

 at the white paper, we view the stronger light of the 

 atmosphere through the same, or any other coloured glass, 

 it is evident, that we can have no fixed standard for white 

 light; the light, which, under different circumstances, we 

 recognize as white, may be formed of the coloured rays 

 in very different portions. 



That this is the case, we have abundant evidence ; the 

 light of the sun, and the light of a lamp, or a candle have 

 been proved, by numerous experiments, to be very different 

 in colour, and, of course, to be composed of the coloured 

 rays in different proportions ; yet, objects which appear 

 white in one of these lights, are recognized as white in the 

 other, and we only discover the difference by bringing 

 them into contrast. — The method of doing this by suffering 

 the light of the sun, reflected by the atmosphere, and the 

 direct light of a candle, to fall upon different parts of the 

 same white paper, is well known. — I lately observed a 

 pleasing variety of this experiment : while holding a sheet 

 of thin writing paper before a fire, the back of the paper 

 being rather obscurely illuminated by the light from an 

 opposite window, I observed the whole of the writing, 

 which was black and turned towards the fire, appeared of 

 a beautiful light blue. I readily discovered that the ink 



