THE SPECTRUM. 



Accidental colours, or ocular spectra, are, so to speak, illusions, and 

 differently-coloured objects will, when our gaze is turned from them, give us 

 different " spectra" or images. For instance, a violet object will, when we 

 turn to a sheet of white paper, give us a yellow "spectrum"; orange will 

 be blue ; black and white will change respectively; red will become a blue- 

 green. From a very strong white light the accidental colours will vary. 



The Solar Spectrum is the name given to the coloured band formed by 

 the decomposition of a beam of light into its elementary colours, of which 

 there are seven. This is an easy experiment. A ray of light can be 

 admitted into a darkened room through 

 a hole in the shutter, and thus ad- 

 mitted will produce a white spot on 

 the screen opposite, as at g in the 

 diagram (fig. 149). If we interpose a 

 prism a triangular piece of glass 

 the "drop" of a chandelier will do we 

 cause it to diverge from its direct line, 

 and it will produce a longer streak of Fig ' M9 Thc Sohr s P ectrum - 



light lower down. This streak will exhibit the prismatic colours, or the 

 "colours of the rainbow"; viz., red (at the top), orange, yellow, green, blue, 

 indigo (blue), and violet last. These are the colours of the Solar Spectrum. 

 The white light is thus decomposed, and it is called mixed light, because 

 of the seven rays of which it is composed. These rays can be again col- 

 lected and returned to the white light by means of a convex lens. 



" White light," said Sir Isaac Newton, " is composed of rays differently 

 refrangible," and as we can obtain the colours of the rainbow from white 

 light, we can, by painting them on a circular plate and turning it rapidly 

 round, make the plate appear white. Thus we can prove that the seven 

 colours make "white" when intermingled. But Newton (1675) did not 

 arrive at the great importance of his experiment. He made a round hole 

 in the shutter, and found that the various colours overlapped each other. 

 But, in i 802, Dr. Wollaston improved on this experiment, and by admitting 

 the light through a tiny slit in the wood, procured an almost perfect spectrum 

 of "simple" colours, each one perfectly distinct and divided by black lines. 



But twelve years later, Professor Fraunhofer made a chart of these lines, 

 which are still known by his name. Only, instead of the 576 he discovered, 

 there are now thousands known to us ! To Fraunhofer's telescope Mr. 

 Simms added a collimating lens, and so the Spectroscope was begun ; and 

 now we use a number of prisms and almost perfect instruments, dispersing 

 the light through each. We have here an illustration of a simple Spectro- 

 scope, which is much used for chemical analysis (fig. I 50). 



In the spectrum we have long and short waves of light, as we have long 

 and short (high and low) waves in music, called notes. The long or low 

 notes are as the red rays, the high notes as the blue waves of light. (Here 

 \ve have another instance of the similarity between light and sound.) But 



