290 SCIENTIFIC AMUSEMENTS. 



when exposed to this kind of influence, of absorbing (so to speak) 

 the invisible light, and emitting it again in a form perceptible by 

 the eye; so that a solution of sulphate of quinine, glass containing 

 the somewhat rare metal uranium, a decoction of horsechestnut 

 bark, and many other substances, will apparently become luminous 

 and emit light when placed in the track of a beam of invisible 

 light of this kind. Here an alteration obviously takes place in 

 the quality of the light, that emitted being not identical in quality 

 with that absorbed; to this property the term fluorescence is 

 applied, it having been first observed with certain kinds of fluor spar. 



Whatever may be the source of light, whether terrestrial or 

 extra-terrestrial, whether natural or artificial, certain points are 

 common to all lights, whilst certain differences exist in particular 

 cases. The ordinary eye distinguishes two qualities in reference 

 to light, viz., brilliancy or illuminating power and colour (vide 

 Chapter XXI). The light of the sun, especially in cloudless 

 tropical climates, the intensely bright electric arc light, and in a 

 somewhat lesser degree the light emitted by intensely heated 

 quicklime (lime light, Expt. 211) and other substances, and by 

 burning phosphorus (Expt. 234), &c., represent sources of light of 

 the highest degree of brilliancy. When none of these or other less 

 brilliant sources of light are active, ordinary objects are invisible to 

 the eye ; but when illuminated in presence of a source of more or 

 less bright white light, such objects become visible, and in many 

 cases show the phenomena of difference in colour ; thus the grass 

 appears green, the poppy flower red, the sunflower yellow, the violet 

 a tint approaching to blue, and so on. Objects thus become 

 visible because they act on the light illuminating them somewhat 

 after the fashion of sulphate of quinine, &c., upon invisible light, 

 in so far as they absorb the light fatting upon them and re-emit it, 

 but not altogether in the same way ; as a rule, they do not emit 

 light of a different quality from .that falling upon them, the differ- 

 ences in colour, &c., resulting from the fact that ordinary bright 

 white light is, so to speak, a conglomeration of all kinds of colours, 

 of which a given object illuminated therewith can re-emit one or 

 more, kinds more easily than the others. 



Thus the grass appears green in sunlight, because sunlight con- 

 tains rays of all kinds of colours, which when mixed together 

 produce upon the eye that effect called " whiteness " (Expt. 350) ; 

 whilst of these different kinds of rays those producing the effect 

 termed " greenness " are re-emitted by the grass much more freely 

 than the others. The poppy appears red for the analogous reason 

 that this flower has the power of re-emitting the rays producing 

 the sensation termed "redness" more freely than the others; and 



