176 TJie Intensity of Transmitted Light. 



the conditions under which the investigation is conducted, 

 leads to a clearer perception of the problem to be solved. 

 For instance, in the theory of heat the imaginary engine of 

 Carnot, which involves conditions incompatible with our 

 received notions of matter has nevertheless assisted in the 

 development and definite expression of an important law in 

 thermodynamics. Also, if mathematicians had waited for 

 the discovery of a perfectly rigid solid, we should not yet 

 have any treatises on statics and dynamics. Also, in the 

 profound treatise of Fourier on heat, the solids considered 

 involve conditions of conductivity and specific heat not 

 exactly fulfilled by physical solids. 



The subject of this paper manifestly admits of several 

 practical applications ; for instance, the quantitative determi- 

 nation of colouring matter undergoing change, in cases 

 where the balance could not be used, on account of the 

 minute quantity of the body to be estimated, or on account 

 of its instability. It may also be applied to the determina- 

 tion of quantities and intensities of light. 



