320 



SCIENTIFIC AMUSEMENTS. 



produce the subsequent impression of blue letters on a white 

 ground ; whilst white letters on an orange ground will give the 

 impression of black letters on a blue ground. This device for 

 attracting attention is utilised in various well known advertise- 

 ments. 



Expt. 355. Coloured Light Examined by the Prism. If 

 bright white sunlight or electric arc light 

 be examined by means of the prism, 

 the spectrum described in Expt. 349 is 

 obtained ; but if a source of some coloured 

 light be substituted for the white beam 

 the spectrum obtained is more or less 

 different. Thus, from a beam of sunlight 

 decomposed by the prism let all light be 

 cut off by means of a screen, except 

 some particular small pencil of coloured 

 rays allowed to pass through a hole in 

 the screen (fig. 157). On allowing this 

 coloured pencil to pass through a second 

 prism it will be deviated in direction 

 in accordance with the law of refraction 

 (Expt. 333), but will not be again split 

 up into rays of different refrangibility, 

 forming a second spectrum. In short, 

 the selected pencil of rays is practically 

 monochromatic ; i.e., it consists of rays 

 of only one colour or one degree of re- 

 frangibility. 



If, however, some other natural or 

 artificial source of coloured light be 

 examined, different results will be 

 noticed in different cases. The light 

 emitted from a flame of spirits of wine and common salt (Expt. 

 245) is naturally monochromatic ; so that if a beam of such light 

 be made to pass through a prism, no coloured spectrum will 

 be obtained, but only an alteration in the direction in which 

 the light travels, due to refraction. Quite a different result 

 will be obtained if a beam of white light be made to pass 

 through a coloured transparent medium (tinted glass, &c.), and 

 then examined by the prism. In this case a spectrum will be 

 obtained, usually showing several colours, but by no means identical 

 with the spectrum obtained from the original beam before passing 

 through the coloured medium ; certain tints will be either entirely 

 absent, or at least very much paler than the others, this being due 



Fig. 161. Zoetrope holder. 



