AND THE THEORY OF LIGHT. 21 



left by any analytical process known.* Again : it is said, 

 "that common daylight appears in comparison with the 

 light of a candle of a purplish hue," He does not now 

 call common daylight white, but he virtually says that 

 the white light of day is purple compared with the abun- 

 dant yellow light of a candle. What, then, is white ? or, 

 what is the light of a candle? 



34. There is no contending against such a method of 

 reasoning as this of Dr. Young's; the premises are con- 

 stantly shifting for want of intelligible elementary prin- 

 ciples. If every new case such as this, in any theory of 

 colour, requires a new hypothesis or some new supposi- 

 tion, to help out an explanation, there is evidently no 

 science, unless conjecture can be dignified with that name. 

 Science requires a general law which shall apply to every 

 case ; and in the illustration of that law we must use lan- 

 guage which can be understood by the ordinary intellect, 

 or it is not axiomatic; we must appeal to experiments 

 which can be seen to be axiomatic or purely analytical. 

 Without axioms we have no basis on which to build our 

 reasoning ; and as the axioms of physical science can only 

 be founded on experiment, to appeal to phenomena which 

 cannot be subjected to experiment, as that of taking away 

 the abundant yellow from the light of a candle, and leav- 

 ing or substituting white, as Dr. Young has done here, is 

 virtually to abandon philosophy to support a theory. 



35. No part of Dr. Young's explanation can be admit- 

 ted as correct, for even the comparison is not of daylight 

 with the light of a candle. The screen is illuminated with 

 the light of day plus the light of the candle, and the sha- 

 dow is a comparison with the light on the screen, not 

 with the light of the candle. The pitch or tone of the 

 luminous ether is raised, and the eye or retina estimates 



* I suspect iudigo and violet, and even purple, are often used as 

 synonymes by writers on light. 



