AND THE THEORY OF LIGHT. 33 



lights one of daylight plus two of candlelight. Here we 

 have the light of day as compared with that of two candles 

 of a dark purple or blue^ and the light of one candle plus 

 the light of day compared with that of two candles of a 

 lighter purple, which appears to be hardly in accordance 

 with the principles of logic. For, according to this method 

 of arguing, if we remove the daylight entirely, the light 

 of one candle should appear to the light of two candles 

 as a grey, of which it would be difficult to convince our 

 senses. 



37. This is a most instructive experiment, one on which 

 the whole theory of light might be based, had it not 

 already received the form generally acquiesced in. But 

 we shall find that it cannot be explained by means of 

 either theory of light. According to the emission theory 

 there should be no change in the colour of the white, for 

 there has been no change in refraction; every thing 

 externally is just as it was, the daylight not being changed 

 by the light of the candle : and according to the uudula- 

 tory theory there should be no change in the white light 

 of day, for there has been no change in the wave of light; 

 its intensity has not been diminished nor increased ; and 

 the undulatory theory makes no allowance for comparison. 

 One wave may be more intense than another, but it does 

 not on that account, — unless on the doctrine of inter- 

 ference, and interference can only exist under special 

 conditions — thereby diminish the intensity of the less 

 intense ; much less can comparison change one wave into 

 another, or, in other words, according to the recognized 

 principles of both theories, decompose the white light into 

 its constituent parts — change the heterogeneous white 

 into the homogeneous blue. If the power of comparison 

 can accomplish so much, why not let it accomplish the 

 whole? why not make it the cause of colour at once? 

 But this is just another instance of departure from or 



