476 



spectrum is observed to be crossed throughout its whole extent with 

 alternate dark and coloured bands, increasing in number and dimi- 

 nishing in magnitude with the thickness of its plate. In the pheno- 

 mena of periodical colours there are three peculiarities demanding 

 notice ; first, that the dark lines change their places by varying the 

 inclination of the plate ; secondly, that two or more lines never co- 

 alesce into one ; and thirdly, that the colour of the luminous bands 

 in the complementary spectrum are the same as those of the original 

 spectrum when the thin plate is perfectly colourless. The author in- 

 stitutes a comparison of these phenomena with those of absorption 

 as exhibited by a solid, a fluid, and a gaseous body ; employing as an 

 example of the first, smalt blue glass ; of the second, the green sap 

 of vegetables ; and of* the third, nitrous acid gas. No connecting 

 link between these phenomena appeared to exist, excepting that 

 both exhibited a divided or mutilated spectrum ; but even this com- 

 mon fact has not the same character in both. The nacreous substance 

 described by Mr. Horner, however, in some cases, when the plates 

 were small, was found to produce bands perfectly identical with those 

 of thin plates j while in other cases the bands were exactly similar to 

 those of coloured media. By employing the iridescent films of de- 

 composed glass, the author obtained combinations of films which gave, 

 by transmitted light, the most rich and splendid colours, surpassing 

 every thing he had previously seen among the colours either of na- 

 ture or of art. These facts have proved that the transmitted colours, 

 though wholly unlike those of thin plates, are yet produced by the 

 same causes, and are residuary, and generally complementary to the 

 sum of the reflected tints. Thus the author has succeeded in com- 

 pletely identifying in their primary features the two classes of facts ; 

 the one resulting from absorption, the other from periodic action. 

 The minor points of difference, namely, the uniformity of the bands 

 and tints of absorbing media at all incidences, and the non-appear- 

 ance of the reflected tints in such media, are endeavoured to be ex- 

 plained by the introduction of several considerations, the complete 

 discussion of which the author reserves for the subject of a future 

 paper. From the phenomena of thin plates, of polarized tints, 

 and of absorption, the existence of a new property of light is deduced, 

 in virtue of which the reflecting force selects out of differently co- 

 loured rays of the same refrangibility rays of a particular colour, al- 

 lowing the others to pass into the transmitted ray 5 a principle not 

 provided for in either of the theories of light to which the phenomena 

 of absorption are ultimately referable, and furnishing an explanation 

 of certain remarkable phenomena of dichroism in doubly refracting 

 bodies, in which rays of the same refrangibility, but of different co- 

 lours, pass into the ordinary and extraordinary pencils. 



A paper was read et On the hereditary instinctive propensities of 

 Animals." By Thomas Andrew Knight, Esq., F.R.S. 



The author adduces, in support of the principle he had advanced 

 in his paper on the economy of bees, namely, that instinctive pro- 

 pensities to the performance of certain actions are transmitted, inde- 



