120 CORRELATION OF PHYSICAL FORCES. 



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 by the air and surrounding 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-chestnuts, or 

 even paper soaked in these latter solutions. Other substances 

 exhibit this effect in different degrees ; 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 imping- 

 ing on certain bodies, become luminous when impinging on 

 others. We might imagine a room so constructed that such 

 emanations 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 alter- 

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

 of rays, might be rendered dark by windows which would be 

 transparent 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 

 as to the emittent body. That which is, or becomes, light 

 when it falls upon one body is not light when it falls upon 

 another. Probably the retinas of the eyes of different per- 

 sons differ to some extent in a similar manner ; and the same 



