ASTRONOMY, ETC., OF SEVENTEENTH CENT. 225 



expressed the phenomena of thin films readily lends itself to that view 

 he expressly states that he does not mean more than that at alternate 

 points in the ray something occurs which in one case disposes it for 

 reflection, in the other for transmission : " what kind of disposition 

 this is whether it consists in a circulating or vibrating motion of the 

 ray, or of the medium, or something else, I do not here inquire." The 

 colours of thin plates have been much discussed since Newton's time, 

 and numberless experiments have been made on this subject, but 

 Newton's measurements have been found wonderfully accurate, and 

 his determinations of the intervals along the rays retain their value, 

 although the facts are explained by another theory of the nature of 

 light. 



By placing liquids instead of air between the glasses, Newton was 

 able to study the colours produced in substances having greater re- 

 fractive power, and he found that in proportion to the refractive power 

 the rings contracted, that is, a less thickness of the film was required 

 to produce them ; and he expressed this, saying that the length of the \ 

 intervals or fits was diminished when the light entered a more refract- 1 

 ing medium. We shall not here be able to indicate all the various 

 speculations connected with light and colours into which Newton 

 entered ; but one of his inferences has often been mentioned as a 

 curious instance of an induction that had to wait more than a century 

 for its verification. Newton had been trying experiments on the re- 

 fractive powers of various transparent substances, and he observed that 

 the refractive power for the most part followed the order of the den- 

 sities of the various bodies. But he observed also a class of highly 

 refractive substances whose power in this respect had no relation to 

 their densities. He remarked that all these substances were what he 

 called "unctuous and sulphureous," that is, inflammable. Finding 

 that the high refractive power of the diamond gave it a position among 

 these bodies, he hazarded the conjecture, which at that time seemed 

 so improbable, that the diamond is in reality a combustible body 

 "an unctuous substance congealed." But although this conjecture 

 has been verified, and phosphorus, a very inflammable substance, has 

 also been found to possess a very high refractive power, no kind of 

 connection between the combustibility and refractive power has been 

 established. 



An unexpected property of light was discovered by a learned Jesuit 

 named FRANCIS MARIA GRIMALDI (1619 1663), and announced to 

 the world in a treatise published in 1665, two years after the author's 

 death. He describes how, having admitted a beam of sunlight into a 

 dark chamber through a pin-hole made in a sheet of lead, he found 

 that the light diverged from the opening in a conical form to a greater 

 degree that it should have done had it simply passed through in straight 

 lines from the sun. He found also that the shadow of a hair placed 

 in this light was larger than could be accounted for by supposing the 



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