NATURAL PHILOSOPHY. 149 



including: those noticed by Sir David Brewster in his papers on " In- 

 ternal Dispersion ;" and has distinguished between " cases of false 

 internal dispersion," or " opalescence," in which the luminous rays are 

 simply reflected from fine particles held in mechanical solution in the> 

 medium, and those of " true internal dispersion," or " fluorescence," 

 as it is termed by Prof. Stokes. By suitable methods of observation 

 the change of refrangibility was detected, as produced not only by 

 transparent fluids and solids, but also by opaque substances ; and the 

 class of media exhibiting " fluorescence " was found to be very large, 

 consisting chiefly of organic substances, but comprehending, though 

 more rarely, some mineral bodies. The direct application of the fact, 

 as we now understand it, to many highly interesting and important 

 purposes, is obvious almost on the first announcement. The facility 

 with which the highly refrangible invisible rays of the spectrum may 

 be rendered visible by being passed through a solution of sulphate of 

 quinine or other sensitive medium, affords peculiar advantages for the 

 study of those rays ; the fixed lines of the invisible part of the solar 

 spectrum may now be exhibited to our view at pleasure. The con- 

 stancy with which a particular mode of changing the refrangibility of 

 light attaches to a particular substance, exhibiting itself independently 

 of the admixture of other substances, supplies a new method of analysis 

 for organic compounds which may prove valuable in organic chemistry. 



DOVE'S THEORY OF LUSTEE. 



SIR DAVID BREWSTER, #t the British Association, explained the 

 theory of Dove respecting the origin of lustre, which was, that the 

 lustre of bodies and particularly the metallic lustre arose from the 

 light coming from the one stratum of the superficial particles of bodies 

 interfering on the eye with the light coming from other and deeper 

 strata, the regular s^ymmetrical arrangement of the particles in these 

 bodies producing effects somewhat analogous to that of mother-of- 

 pearl. But the opinion which Sir David himself seemed to incline to, 

 was, that since we know from the phenomena of very thin metallic 

 leaves that lights of very different colors are transmitted through strata 

 of different kinds of matter and of different thicknesses, and since 

 from the different refrangibility of lights of these colors, the same lens 

 will not bring them to a focus at the same distance, metallic lustre 

 was caused by the effort used to accommodate the eye to the distinct 

 vision of these colors. 



ART OF SEEING THE INTERIOR OF THE EYE BY THE EYE ITSELF. 



THE following paragraph has recently been published in several 

 journals, relative to a discovery said to have been made by M. 

 Andraud, an eminent French engineer. 



" Some attention has been excited by the alleged discovery, by a 

 French engineer of some celebrity named Andraud, of some means of 

 seeing the air. If, says he, you take a piece of card, colored black, of 



