162 THEORY OF LIGHT AND COLOURS. 



Air bladders of Dr. Darwin fuppofes, that the air in the air bladders of vc- 

 contaia ai^worfe £ et ables (erve to oxigenate the feed. Tlie air of the air blad- 

 than that of the ders of cardiofpermum halicacabum, ftaphylia trifoliata, coin- 

 atnco p ere. tea arDOre f cens> anc j fophora auftralis being examined, was 

 found to be a little worfe than the air of the atmolphere. 



III. 



On the Theory of Light and Colours, ify Thomas Young, 

 M. D. F. R. S. Profeffor of Natural Phihfophy in the Royal 

 Infiitution. 



(Concluded from page 90.^ 



PROPOSITION IV. 



Prop. IV. Tar- When an Undulation arrives at a Surface which is the Limit of 

 undubUong°at Mediums of different Denjities, a partial Reflection takes place, 

 the confine of proportionate in Force to the Difference of the Denjities. 



mediums differ- 



JL HIS may be illuflrated, if not demonflrated, by the ana- 

 logy of elaftic bodies of different fizes. " If a fmaller elaftic 

 body ftrikes againft a larger one, it is well known that the 

 fmaller is reflected more or lefs powerfully, according to the 

 difference of their magnitudes : thus, there is always a re- 

 flection when the rays of light pafs from a rarer to a denfer 

 ftratum of ether ; and frequently an echo when a found ftrikes 

 againft a cloud. A greater body ftriking a fmaller one, pro- 

 pels it, without lofing all its motion : thus, the particles of a 

 denfer ftratum of ether, do not impart the whole of their mo- 

 tion to a rarer, but, in their effort to proceed, they are re- 

 called by the attraction of the refracting fubflance with equal 

 force ; and thus a reflection is always fecondarily produced, 

 when the rays of light pafs from a denfer to a rarer ftratum." 

 (Phil. Tranf. for 1800, p. 127.) But it is not abfolutely ne- 

 ceffary to fuppofe an attraction in the latter cafe, fince the 

 effort to proceed would be propagated backwards without it, 

 and the undulation would be reverfed, a rarefaction returning 

 in place of a condenfation ; and this will perhaps be found 

 moft confident with the phenomena, 



PROPOSl- 



