54 Britain's Heritage of Science 



deviation, or the refrangibility, as we should now call it, 

 is perfectly definite for each ray, and is intimately connected 

 with its colour. Having once separated a ray of definite 

 colour, no further refraction will alter that colour, and it 

 will continue to retain the same properties. As one of 

 the results of this discovery it became apparent that a lens 

 cannot form a perfect image of an object, because different 

 colours are not brought together at the same focus. This 

 appeared to Newton to be such a serious and irremediable 

 defect of telescopes with glass objectives, that he set himself 

 to construct an instrument in which the principal lens is 

 replaced by a mirror. At the request of the Royal Society, 

 who had heard of his telescope, Newton forwarded the 

 instrument to its secretary in December, 1671, with the 

 result that in January of the succeeding year he was elected 

 a Fellow of the Society. The idea of reflecting telescopes 

 had, as already mentioned, previously occurred to Gregory, 

 whose proposal differed, however, essentially from that of 

 Newton in the manner in which the rays were ultimately 

 brought to the observer's eye. 



Newton's name is attached to the coloured rings seen 

 when two slightly curved surfaces of glass are brought 

 together, so that there is a thin circular wedge of air formed 

 near the point of contact. The explanation of these rings 

 presented considerable difficulties, especially with the theory 

 of light adopted by Newton. Though cognisant of the wave- 

 theory of light, which, as shown by Huygens, could explain 

 its propagation and refraction, Newton had good grounds for 

 not accepting it. He saw that the analogy of sound which 

 had been invoked in its favour broke down when applied 

 to the formation of shadows. Sound after passing through 

 an opening spreads in all directions, while light apparently 

 follows a straight course. In other words, sound can turn 

 a corner, while light seems unable to do so. More than a 

 century later, Fresnel gave the correct explanation of the 

 apparent discrepancy, showing that when the experimental 

 conditions were made to correspond, the analogy was main- 

 tained. It is necessary for the purpose that the relation 

 between the size of the aperture and the length of the wave 

 should be the same, and as the waves of light are very short, 



