222 HISTORY OF SCIENCE. 



person who, previous to Newton, clearly expressed the idea of gravi- 

 tation. In 1666 he describes some experiments undertaken to de- 

 termine whether any variation in a weight of bodies is found when 

 the bodies are at different distances from the earth's centre and he 

 was led to the idea of measuring the force of gravity by observing at 

 different altitudes the rate of a pendulum clock. In a work pub- 

 lished in 1674 he says, "I shall hereafter explain a system of the 

 world differing in many particulars from any yet known, and depend- 

 ing on the three following suppositions : first, that all celestial bodies 

 whatsoever have an attractive or gravitating power by which not only 

 their parts are attracted to their own centre, but they mutually attract 

 each other within the sphere of their activity. The second supposition 

 is that all bodies put into a direct simple motion will so continue to 

 move in a straight line till they are deflected by some other power, 

 and made to move in a circle, ellipse, or other curve. The third sup- 

 position is that the attractive powers are the more powerful the nearer 

 is the attracted body." He adds that he has not experimentally ex- 

 amined what law regulates the increase of the attractive power, but 

 that he thinks an investigation into the subject would be of the greatest 

 utility to astronomers, 



When Newton presented his reflecting telescope to the Royal So- 

 ciety, Hooke criticised the instrument with undue severity, and boasted 

 that he was himself acquainted with an infallible method of giving the 

 utmost perfection to all optical instruments. The fact is that he had ob- 

 served certain phenomena, which we shall presently mention, and from 

 these he had come to the conclusion that light depended upon undula- 

 tions of a highly elastic medium. The theory of colour which Hooke 

 deduced from his speculations, was that there exist but two colours, 

 ra/and violet, which correspond, as he imagined, to the two sides of a 

 wave. Newton, on the other hand, preferred to regard light as a subtile 

 matter thrown off from luminous bodies, but he pointed out that his 

 doctrine of colours did not depend upon either one theory or the 

 other. 



Hooke soon afterwards laid before the Royal Society some curious 

 observations on the colours produced by thin transparent films, such 

 as soap-bubbles, and the excessively narrow layer of air between two 

 plates of glass nearly in contact. He perceived that the colour de- 

 pended in some way upon the thickness of the transparent film, but 

 all h'is attempts to discover the relation between the thickness of the 

 film and the colour produced were unsuccessful. He found it pos- 

 sible to split mica into films of extreme thinness, which gave divers 

 brilliart colours. One film, for instance, gave a yellow colour, another 

 a blue colour, but the two together produced a deep purple. The 

 plates which produced these colours being so thin that at least twelve 

 thousand of them would be required to make up the thickness of one 

 inch, Hooke found it impossible, by any contrivance with which he 



