264 SCIENCE PROGRESS 



attention to the development of the reflecting telescope. We 

 know from the Opticks he worked with a hollow prism filled 

 with water, which should have given a spectrum only half as 

 long as his glass prisms did, and Lucas in Liege, who worked 

 with a crown glass prism, insisted that his spectrum was only- 

 three times the angular diameter of the sun in length, not 

 five times as Newton found it. Why Newton should have per- 

 sisted in error has always seemed strange, and Glazebrook has 

 suggested in his biography in the Encyclopedia Britannica 

 that possibly the water in Newton's hollow prism contained 

 sugar of lead in solution ; this would increase the length of the 

 spectrum, and make it liker the spectrum given by the glass 

 prisms. We know that Newton sometimes worked with sugar 

 of lead solutions. 



But was the whole affair not merely a matter of strong pre- 

 conceived ideas ? The determination of the variation of index 

 of refraction with colour follows closely on the comparison of 

 the spectrum with the musical scale, both in the Opticks and 

 the Lectiones Optical. If the arrangement of the colours in 

 the spectrum bore a fixed relation to the musical scale, it was 

 obviously independent of the material of the prism. Thus, 

 being indebted to Kepler for the idea that there must be a 

 simple numerical law, and taking his stand on his own measure- 

 ments, Newton perhaps believed that there were theoretical 

 reasons demanding that the dispersion of all media should be 

 the same. And the experimental evidence forthcoming was 

 not strong enough to make him give up this belief. 



