LIGHT: COLOUR 



299 



170. Absorption Colours. The only real standard of 

 colour being the composition of its spectrum, many most 

 instructive experiments may be made to show that natural 

 colours are caused by the suppression of certain colours through 

 absorption by the molecules of the substance. The most con- 

 venient arrangement is that in fig. 166, the objective of the 

 lantern being removed and replaced by a perpendicular slit, 

 either adjustable in brass, or it may be cut in a cap of black card. 

 (We shall simply speak of the slit on the nozzle in future.) 

 Through this the light is sent ' parallel,' and focussed by the 

 loose lens F, beyond 

 which is the prism 

 p throwing the spec- 

 trum B c. Over half 

 the slit a coloured 

 glass may be held 

 suppose a deep red. 

 It will be seen that 

 more or less of the 

 spectrum is cut out 

 or obstructed, as 

 from c to A, and that 

 the glass is only 

 transparent to the 

 red rays. The light is no brighter in the red, and therefore 

 no other light is turned into red ; it is simply that the other 

 colours are cut out from the full spectrum. Other coloured 

 glasses will show the same thing ; and it will be observed that 

 some glasses cut out bands in the spectrum. Very instructive 

 pieces of various colours can be selected with a small pocket 

 spectroscope, and these glasses will show by their composition 

 how mixed most colours really are, as we find them, and 

 how different is a really pure coloured light, of a definite 

 refrangibility, from the light reflected or transmitted by 

 ordinary pigments or other coloured substances, which is 



FIG. 166. -Absorption 



