ILLUMINATION. 291 



similarly in all other cases. If the grass be illuminated by light 

 containing no green rays, it will not appear green any longer, but 

 dingy grey or even darker ; similarly, if the poppy be illuminated 

 by light containing no red rays, it will no longer appear red ; and 

 so on in other cases. The reason for the peculiar appearances 

 seen in Expt. 245, where the light used is that of spirits of wine 

 tinted by a salt of sodium (such as common salt, or chloride of 

 sodium), is that the light employed is purely yellow, so that objects 

 which in white light would appear of other colours cannot show 

 these colours when exposed to the sodium spirit flame, because 

 rays of those colours are not contained in the light for them to 

 absorb and re-emit. 



The light emitted from the surface of a given object, not self- 

 luminous but illuminated from other sources, is spoken of as 

 scattered light, being propagated equally in all directions, so that 

 wherever a beholder may be situated this scattered light will 

 equally meet his eye, so as to enable him to perceive the object in 

 question ; if the object possesses the power of scattering all kinds 

 of coloured rays to an equal extent, it will appear white when 

 illuminated by white light ; or, if illuminated by any particular 

 coloured light, it will appear of that same colour ; the larger the 

 proportion of the light originally falling upon or incident on the body 

 from the source of illumination is thus scattered the brighter will 

 the object appear, the various effects of light and shade (apart 

 from colour) in producing whiteness, greyness, and blackness of 

 different objects simply depending upon the relative powers of thus 

 scattering incident light possessed by the objects respectively. 

 Thus, a sheet of white note paper, a grey felt hat, and a black 

 cloth coat may be all three equally illuminated, but they scatter the 

 incident light to very different extents ; the paper scatters largely, 

 the felt hat but little, and the cloth coat practically not at all. 



Unless light meet with substances exhibiting differences in 

 their chemical and physical nature or conditions whilst passing 

 onwards, no scattering takes place. A chamber filled with air or 

 any other gas or vapour absolutely free from all suspended solid 

 particles, such as motes, or liquid vesicles (minute fog or water 

 spray, &c.), appears perfectly dark when beams of the strongest 

 light are passed through it ; if, however, liquid or solid particles 

 be present, these more or less absorb and scatter the light, so that 

 the track of the beam of light is more or less illuminated and 

 rendered visible by this action, which thus serves as a physical 

 test of the purity of a given atmosphere as regards particles of 

 suspended matter, such as ordinary dust and germs of plant life, 

 disease germs, and similar organised matters. 



