Fourth Series. — Hough Surfaces. 81 



sisting of longer undulations, the inequalities of the surface 

 first becoming insignificant for red light. 



25. In this I was not deceived. My purpose not being to 

 investigate fully the subject of diffuse reflexion, I confined my 

 attention to the establishment of the general fact. Employ- 

 ing an apparatus which I have not yet described, but which 

 bears a great analogy to that figured in the Society's Trans- 

 actions, vol. xiv. pi. xiii., and described in art. 51 of the 

 Third Series, I observed the intensity of reflexion of heat 

 from different sources at a single polished surface of flint-glass, 

 and at a similar surface depolished with emery. I obtained 

 at considerable incidences the following striking results as to 

 the increased susceptibility of heat to be regularly reflected at 

 a rough surface, when it is of low temperature or refrangibility. 



Ratio of the Intensities of Heat reflected by a Polished and a 

 Rough Surface of Flint-Glass. 



So far then the character of the action of depolished sur- 

 faces is consistent. The stifling effect (which diminishes both 

 the reflected and refracted ray) of a rough or laminated surface, 

 diminishes with the refrangibility of the incident heat. That 

 the same thing takes place in the reflexion of light we know; 

 it is probable that it does so in its transmission likewise, though 

 this has not been so distinctly observed. Most impure sub- 

 stances transmit a ruddy gleam, vapour of water does so when- 

 ever it is not colourless*, and every practical optician knows, 

 that in a great majority of media the violet end of the spec- 

 trum is first absorbed. 



[To be continued.] 



XV. Notices respecting New Books. 

 On the Theory of the Moon, and on the Perturbations of the Planets. 

 Part IV. [With a "Note on the Calculation of the Distance of a 

 Comet from the Earth."] By J. W. Lubbock, Esq., Treas. R.S., 

 Vice- Chancellor of the University of London, &c. &c. Lond. 1840. 

 8vo. Pp. xiv., 355-417, and 1-6. 



I 



N the former parts of this work (the first of which was noticed 

 in our fourth volume, p. 218) the author endeavoured to explain 



* Edinburgh Transactions, vol. xiv. p. 371. 

 Phil. Mag. S. 3. Vol. 19. No. 121. July 1841. 



G 



