344 Sir David Brewster on the Polarization of Light 



not avoid calling attention to the remarkable circumstance which 

 has frequently presented itself to me, of a great change in the 

 relative visibility of Fraunhofer's lines when seen at different 

 periods. There are times at which the strong lines seen in the 

 red ray are so feeble that the eye can barely catch them ; and 

 then again they come out as dark as though marked in India 

 ink on the paper. During these changes the other lines may or 

 may not undergo corresponding variations. The same observa- 

 tion equally applies to the blue and yellow rays. It has seemed 

 to me that the lines in the red are more visible as the sun ap- 

 proaches the horizon, and "those at the more refrangible end of 

 the spectrum are obvious in the middle of the day." 



The appearance of the spectrum, as well as its chemical action, 

 varies with the hour of the day. MM. Favre and Silbermann, 

 in their memoir " On the Quantities of Heat disengaged in Che- 

 mical and Molecular Actions," published in the Annates de 

 Chimie, 1853, vol. xxxvii. p. 500, have given a projection of three 

 curves furnished by their own observations, and one derived from 

 mine. From the discussion of these results they deduce that 

 there is a connexion between the absorptive action of the atmo- 

 sphere, due to vapour contained in it in greater quantity in the 

 afternoon, and the chemical power of the different spectrum-rays. 

 They suggest that my experiments must have been made after 

 midday, as was in fact the case. 



I intended to add some remarks respecting the so-called recent 

 discovery of the variations of the spectrum and its lines when 

 substances are burnt at different temperatures, but, fearing, that 

 you will grow tired of these reclamations, will content myself with 

 the hope that those of your readers who are occupying themselves 

 with the spectroscope will not think their time wasted if they 

 consult the pages of old Numbers of the Philosophical Magazine. 



Yours truly. 



University, New York, John W. Draper. 



March 26, 1863. 



XLVII. On the Polarization of Light by Rough and White Sur- 

 faces. By Sir David Brewster, K.H., D.C.L., F.R.S* 



THE laws of the polarization of light when reflected from 

 the surfaces of solids and fluids, and when refracted and 

 transmitted by translucent and transparent bodies, have been 

 successfully investigated ; but no experiments, I believe, have 

 been made on the polarization of light by rough or unpolished 

 surfaces, such as ground glass, painted surfaces, pounded glass, 



* From the Transactions of the Royal Society of Edinburgh, vol. xxiii. 

 part 2. Communicated by the Author. 



