NEWTON AND THE COLOURS OF THE SPECTRUM 253 



from blue and violet. In making out a list of the seven most 

 important colours they would have prior claims to indigo. 



It was in the year 1666 that Isaac Newton carried out at 

 Cambridge the experiments on the decomposition of white 

 light by a prism which were to inform us as to the true nature 

 of colour. As source of light he used the brightest of all possible 

 sources, namely, the sun. Its rays were admitted into a dark- 

 ened room through a hole in a shutter. In the first experiment 

 on the spectrum described in the Opticks, i.e. expt. 3, Bk. I. 

 Part 1, this hole was circular and about \ in. in diameter. The 

 room thus acted as a pinhole camera, and an image of the sun 

 in its natural colours would under ordinary circumstances 

 fall on the opposite wall of the room. If, however, a glass prism 

 was placed inside the room close up to the hole, with its re- 

 fracting edge horizontal and pointing downwards, so as to 

 receive the rays, they were refracted, and the image of the 

 sun appeared higher up on the wall, as is shown in the figure, 



which is taken from the Opticks. At the same time it changed 

 its appearance. Instead of appearing as a single yellow disc, 

 when it was received on a sheet of white paper it became a 

 vertical strip with straight sides and semi-circular ends. The 

 strip was 18} ft. from the prism. Its breadth was 2| in., just the 

 same as the diameter of the image would have been had the 

 prism been taken away. If the prism was rotated about a 

 horizontal axis, the image moved down and then up the wall, 

 and its length altered, being a minimum when the image was 

 as far down as possible. The whole length then was io£ in. 

 and the length of the straight sides was 8 in. The strip was 

 coloured, the upper end being violet, the lower end being red, 

 and the intermediate portion being blue, green, and yellow. 

 In this way Newton discovered the prismatic spectrum, 



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