EPOCH OF YOUXG AXD FBESXEL. 91 



unimportant, that the partitions usually employed are not impermeable 

 to sound, as opake bodies are to light. lie observes that the sound 

 does not all come through the aperture ; for we hear, though the 

 aperture be stopped. These were the main original points of attack 

 and defence, and they continued nearly the same for the whole of the 

 last century ; the same difficulties were over and over again proposed, 

 and the same solutions given, much in the manner of the disputations 

 of the schoolmen of the middle a^es. 



O 



The struggle being thus apparently balanced, the scale was natural- 

 ly turned by the general ascendancy of the Newtonian doctrines ; and 

 the emission theory was the one most generally adopted. It was still 

 more firmly established, in consequence of the turn generally taken 

 by the scientific activity of the latter half of the eighteenth century ; 

 for while nothing was added to our knowledge of optical ,aws, the 

 chemical effects of light were studied to a considerable extent by 

 various inquirers ; 13 and the opinions at which these persons arrived, 

 they found that they could express most readily, in consistency with 

 the reigning chemical views, by assuming the materiality of light. It 

 is, however, clear, that no reasonings of the inevitably vague and 

 doubtful character which belong to these portions of chemistry, ouo-ht 

 to be allowed to interfere with the steady and regular progress of in- 

 duction and generalization, founded on relations of space and number, 

 by which procedure the mechanical sciences are formed. We reject, 

 therefore, all these chemical speculations, as belonging to other sub- 

 jects ; and consider the history of optical theory as a blank, till AVC 

 arrive at some very different events, of which we have now to speak 



13 As Scheele, Selle, Lavoisier, De Luc, Eichter, Leonhardi, Gren, Girtanner, 

 Link, Hagen, Voigt. Ds la Metherie, Scherer, Dize, Brugnatelli. See Fischer, 

 rii. p. 20. 



