292 SCIENTIFIC AMUSEMENTS. 



Besides the light thus scattered, in virtue of which objects 

 generally become visible in daylight or when artificially illumin- 

 ated, certain kinds of objects affect incident light in other ways ; 

 thus a flat polished silver surface reflects the light, the light thus 

 reflected being different from scattered light in that it is propa- 

 gated only in one direction, dependent solely on a certain geometrical 

 consideration embodied in the law of reflection (Expt. 332). A 

 pane of windowglass, on the other hand, not only reflects some of 

 the light incident upon its surface, in addition to scattering a part 

 thereof, but also allows a considerable portion to pass inwards into 

 the glass from the outside exposed to the illuminating influence, 

 of which portion a considerable fraction emerges again at the other 

 side of the pane ; so that part of the original light is scattered, 

 part reflected, and part absorbed in the glass, whilst the remainder 

 passes through. 



Bodies which thus allow light to pass through are said to be 

 transparent. If the different coloured rays of incident white 

 light are all equally absorbed in passing through, the transmitted 

 light is also white, but of diminished brilliancy in proportion to 

 the amount of the original light lost by scattering, reflection, and 

 absorption conjointly. If, on the other hand, certain rays are 

 absorbed more readily than others, the light that passes through is 

 defective in these rays as compared with white light, and con- 

 sequently produces a sensation of colour, the character of which 

 depends upon the nature of those rays that pass through most 

 readily. Thus, a piece of glass stained dark yellow (such as is used 

 for photographic purposes) absorbs all rays producing blue and 

 violet coloured effects (and also, and more particularly the chemical 

 or actinic invisible rays Expt. 349), so that the light that does 

 pass through is deprived of these rays, and produces on the eye 

 much the same effect as the light reflected or scattered from a 

 yellow object. On the other hand, glass tinted deep blue violet 

 with cobalt, stops yellow and green rays readily, but allows blue 

 and violet rays, also the actinic ones, to pass pretty readily. Alum, 

 on the other hand, allows light of all colours to pass through 

 equally readily, and hence does not appear of any particular 

 colour; but it exerts a most powerful absorbent action on in 

 visible rays of low refrangibility, or heat rays (Expt. 377). 



When light passes into a transparent body, or optical medium, 

 from another medium outside, it generally suffers an alteration in 

 the direction in which it is propagated, somewhat after the fashion 

 of reflected light, but in accordance with a different law, known 

 as the law of refraction (Expt. 333), or alteration of direction in 

 passing into one transparent medium from another. The law of 



