C. GENERAL PHYSICS. 141 



coarser gratings are much the best in respect of definition, and 

 that the same difference is observed in the copies. He has also 

 attempted to take copies of copies, but with indifferent suc- 

 cess, even when the performance of the first copy was not 

 perceptibly inferior to that of the original. Gratings may 

 be copied without the aid of photography, by simply taking 

 a cast. He has obtained a fair result by allowing filtered 

 gelatine to dry, after being poured on the 3000-line Nobert 

 srratinff. This method, however, is attended with much more 

 risk to the original. In considering the theory of the action 

 of these gratings on a beam of light, Lord Rayleigh shows 

 that, in order that the grating may be able to resolve the D 

 line of the solar spectrum, there must be no systematic irregu- 

 larity to the extent of a thousandth part of the interval be- 

 tween the successive lines, though single lines may, of course, 

 be out of position to a much larger amount. 7-4, XLYIL, 

 81-193. 



THE LAWS OF THE REFLECTION OF LIGHT. 



The question of the laws of reflection of light, as far as 

 regards the intensity of the reflected light, has been the sub- 

 ject of investigation, both theoretical and experimental, by 

 Professor Pickering, of Boston. The quantity of light re- 

 flected increases with the co-efficient of refraction, a law which 

 explains many familiar phenomena such as the brilliancy of 

 the diamond, which is practically understood in the diamond 

 mines, where it is customary to distinguish the rough gem 

 from the quartz pebbles by immersing them in water, when 

 the pebbles become scarcely distinguishable from each other 

 and from the water, while the diamond becomes prominently 

 visible. In order to compare the theoretical formulae of Fres- 

 nel with the facts of nature, Professor Pickering devised a 

 modification of the Savart polarimeter, and has applied his 

 instrument, first of all, to the study of the polarization of the 

 light of the sky. All his observations point to one very re- 

 markable result : namely, that the polarization is the same 

 for a given solar distance for any meridian distance ; or, in 

 other words, is the same for all points equally distant from 

 the sun. The variations from this law are slight, and may 

 be ascribed partly to errors of observations, and partly to 

 real irregularities in the atmosphere. The simplest explana- 



