30 colour-sight and colour-blindness 



Light and its Composition. 



Sir Isaac Newton (1642-1727) said " White light is com- 

 posed of rays differently refrangible." 



If a beam of sunlight be admitted to a dark room 

 through a very narrow slit in a shutter and received on the 

 edge of a triangular prism — the drop of a chandelier will do — 

 it will be bent from its straight course and may be projected on 

 a screen as a brilliant rainbow-like band of many coloured 

 lights — the spectrum. The beam has been decomposed or dis- 

 persed by refraction and the seven so-called primary colours 

 result — red, orange, yellow, green, blue, indigo and violet ; red 

 being the colour of least refraction, violet of most. This 

 division into seven is purely arbitrary, and Professor Leslie 

 suggests that Newton " in the choice of that number was 

 apparently influenced by some lurking disposition towards 

 mysticism." 



If the solar beam be examined through a spectroscope, the 

 spectrum obtained is broken up by numerous fine transverse 

 lines, named after their discoverer Frauenhofer's (1787-1826) 

 lines. The more prominent of these are lettered, A being at 

 the extreme end of the red and H of the violet, and, as their 

 positions are fixed, they are the standards by which places in 

 the spectrum are located and referred to. In the spectrum 

 mentioned, the red, orange, yellow and green are crowded up, 

 making together half the luminous band, while the blue, indigo 

 and violet are extended, and make the other half. 



Spectra are produced by other means than prismatic refrac- 

 tions ; by a diffraction grating, the polariscope, phosphorescent and 

 fluorescent bodies, thin films and the action of coloured bodies. 



The diffraction spectrum is not so brilliant nor so pure as 

 the refraction one ; but the distances between the colours are in 

 proportion to their wave lengths, hence it is called the normal 

 spectrum and yellow is in the middle of it. 



The spectrum has been divided into a thousand (1000) 

 parts ; the positions of the colours and the spaces occupied by 

 them have been defined and the fixed lines of Frauenhofer have 

 their allotted places, even to decimal points. Then as difference 

 of wave length gives rise to difference of colour, these waves 

 have been calculated to the ten-millionths of a millimetre and 

 so also with the fixed lines. 



Sir David Brewster (1781-1868) reduced Newton's pri- 

 maries to three : — red, yellow, and blue, and this seemed 



