LIGHT. in 



image of the exposed portion of the engraving is reproduced : 

 similar results are produced by mottled marble exposed to the 

 sun ; an invisible tracing on paper by a fluorescent body, sul- 

 phate of quinine, is, after insolation, reproduced on photogra- 

 phic paper, &c. Insolated paper retains the power of 

 producing an impression for a very long period, if it is kept in 

 an opaque tube hermetically closed. 



It is right to observe that these effects are supposed by 

 many to be due to chemical emanations proceeding from 

 the substances exposed to the sun, and which are be- 

 lieved to have undergone some chemical change by this 

 exposure ; this \vould still be a molecular effect, but it is 

 desirable to await further experiment before forming a de- 

 cided opinion. 



The analogies in the progression of sound and light are 

 very numerous : each proceeds in straight lines, until inter- 

 rupted ; each is reflected in the same manner, the angles of 

 incidence and reflexion being equal ; each is alternately nulli- 

 fied and doubled in intensity by interference ; each is capable 

 of refraction when passing between media of different density : 

 this last effect of sound, long ago theoretically determined, 

 has been experimentally proved by Mr. Sondhauss, who con- 

 structed a lens of films of collodion, which, when filled with 

 carbonic acid, enabled him to hear the ticking of a watch 

 placed at one focus of the lens, the ear of the experimenter 

 being at the opposite focus. The ticking was not heard when 

 the watch was moved aside from the focal point, though it 

 remained at an equal distance from the ear. An experiment 

 of M. Dove seems, indeed, to show an effect of polarisation of 

 sound. 



The phenomena presented by heat, viewed according to 

 the dynamic theory, cannot be explained by the motion of an 

 imponderable ether, but involve the molecular actions of ordi- 

 nary ponderable matter. The doctrine of propagation by un- 

 dulations of ordinary matter is very generally admitted by 

 those who support the dynamical theory of heat ; but the 

 analogies of the phenomena presented by heat and light are 

 so close, that I cannot see how a theory applied to the one 

 agent should not be applicable to the other. When heat is 



