io4 CORRELATION OF PHYSICAL FORCES. 



the light first proceeds. The recent experiments of Sir John 

 Herschel and Mr. Stokes show that radiant impulses, which, 

 falling on certain bodies, give no effect of light, become 

 luminous when falling on other bodies. 



Thus, let ordinary solar light be refracted by a prism (the 

 best material for which is quartz), and the spectrum received 

 on a sheet of paper, or of white porcelain ; looking on the 

 paper, the eye detects no light beyond the extreme violet 

 rays. If, therefore, an opaque body be interposed so as just 

 to cut off the whole visible spectrum, the paper would be 

 dark or invisible, with the exception of some slight illumina- 

 tion from light reflected from dust in the air and from sur- 

 rounding bodies. Substitute for that portion of the paper 

 which was beyond the visible spectrum a piece of glass 

 tinged by the oxide of uranium, and the glass is perfectly 

 visible ; so with a bottle of sulphate of quinine, or of the 

 juice of horse-chesnuts, or even paper soaked in these latter 

 solutions. Other substances exhibit in different degrees this 

 effect, which is termed fluorescence ; and among the sub- 

 stances which have hitherto been considered perfectly analo- 

 gous as to their appearance when illuminated, notable differ- 

 ences are discovered. Thus it appears that emanations which 

 give no impression of light to the eye, when impinging on 

 certain bodies, become luminous when impinging on others. 

 We might imagine a room so constructed that such emana- 

 tions alone are permitted to enter it, which would be dark or 

 light according to the substance with which the walls were 

 coated, though in full daylight the respective coatings of the 

 walls would appear equally white ; or, without altering the 

 coating of the walls, the room exposed to one class of rays 

 might be rendered dark by windows which would be trans- 

 parent to another class. 



If, instead of solar light, the electrical light be employed 

 for similar experiments, an equally striking effect can actually 

 be produced. A design, drawn on white paper with a solu- 

 tion of sulphate of quinine and tartaric acid, is invisible by 

 ordinary light, but appears with beautiful distinctness when 

 illuminated by the electric light. Thus, in pronouncing upon 

 a luminous effect, regard must be had to the recipient as well 



