CH, XX. THE ACHROMATIC TELESCOPE. 169 



the artificial colours are not pure, and also because it is 

 difficult to paint each colour in the proper proportion. 



But now that we have proved that light is broken up 

 into colours in passing through a denser medium, you may 

 perhaps ask how it is that we do not see coloured rays when- 

 ever we look at the sun through glass or any other trans- 

 parent substance. The reason is that when the two sides of 

 the glass sue parallel (that is, lie always at the same distance 

 from each other), the ray of light is bent just as much in 

 going out from the glass into the air as it was when it came 

 in from the air into the glass, and so it remains just as it was 

 at first. When the two sides are not parallel, as in a rounded 

 lens, colours do appear in the thin edges of the glass, and 

 these used to be very troublesome in telescopes and micro- 

 scopes. Newton thought that they could never be got rid 

 of, for he did not know that light is spread out or dispersed 

 more in one kind of glass than in another. But two years 

 after his death, in 1729, Mr. Chester More Hail, of Essex, 

 found that two kinds of glass (flint-glass and crown-glass) 

 disperse light differently, so that when you put them together 

 they correct each other, and the coloured rays at the edges 

 are blended into white light. Telescopes and microscopes 

 which are made in this way are called achromatic (from a, 

 without ; chroma, colour). A patent for such instruments was 

 taken out by a Mr. Dollond in 1757, and he probably in- 

 vented them without having heard of Mr. Hall's discovery. 



It would require a whole volume to give you all Newton's 

 investigations into the nature of light, and his experiments 

 on the coloured rings of the soap-bubble and other trans- 

 parent substances. His work on Optics was read before the 

 Royal Society in 167 1 and 1672, but the ideas were so new 

 that many clever men, who should have known better, 

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