Discrimination of Organic Bodies by their Optical Properties. 393 



ance [these were shown on a screen] . Now, when the green light 

 reflected from the crystals is analyzed by a prism, there are observed 

 bright bands, indicating maxima of reflecting power, corresponding 

 in position to the dark bands in the light transmitted by the solu- 

 tion. The fifth bright band, indeed, can hardly, if at all, be made 

 out ; but the corresponding dark band is both less strong than the 

 others and occurs in a fainter part of the spectrum. When the]light 

 is reflected at a suitable angle, and is analyzed both by a Nicol's 

 prism, placed with its principal section in the plane of incidence, 

 and by an ordinary prism, the whole spectrum is reduced to the 

 bands just mentioned. The Nicol's prism would under these cir- 

 cumstances extinguish the light reflected from a vitreous substance, 

 and transmit a large part of the light reflected from a metal. Hence 

 we see that as the refrangibility of the light gradually increases, the 

 substance changes repeatedly, as regards the character of its reflect- 

 ing power, from vitreous to metallic and back again, as the solution 

 (and therefore it may be presumed the substance itself) changes 

 from moderately to intensely opake, and conversely. 



These considerations leave little doubt as to the chemical state of 

 the copper present in a certain glass which was exhibited. This 

 glass was coloured only in a very thin stratum on one face. By 

 transmission it cut off a great deal of light, and was bluish. By 

 reflexion, especially when the colourless face was next the eye, it 

 showed a reddish light visible in all directions, and having the 

 appearance of coming from a fine precipitate, though it was not 

 resolved by the microscope, at least with the power tried. It evi- 

 dently came from a failure in an attempt to make one of the ordinary 

 red glasses coloured by suboxide of copper, and the only question 

 was as to the state in which the copper was present. It could not 

 be oxide, for the quantity was too small to account for the blueness, 

 and in fact the glass became sensibly colourless in the outer flame 

 of a blowpipe. Analysis of the transmitted light by the prism 

 showed a small band of absorption in the place of the band seen in 

 those copper-red glasses which are not too deep ; and therefore a 

 small portion of copper was present in the state of suboxide, i. e. 

 a silicate of that base. The rest was doubtless present as metallic 

 copper, arising from over-reduction in the manufacture ; and accord- 

 ingly the blue colour, which would have been purer if the suboxide 

 had been away, indicates the true colour of copper by transmitted 

 light, quite in conformity with what we have seen in the case of 

 gold. Hence, in both metals alike, the absorbing and the reflecting 

 powers are, on the whole, greater for the less than for the more 

 refrangible colours, the law of variation with refrangibility being of 

 course somewhat different in the two cases. 



Time would not permit of more than a very brief reference to the 

 second property to which the speaker had referred as useful in tra- 

 cing substances in impure solutions — that of fluorescence. The 

 phenomenon of fluorescence consists in this, that certain substances, 

 when placed in rays of one refrangibility, emit during the time of 

 exposure compound light of lower refrangibility. When a pure 



