42 THE PRINCIPLES OF SCIENCE. 



splendid experiments on the density of the earth. The 

 accuracy of his results entirely depended upon the eli- 

 mination of all disturbing influences, so that the oscillation 

 of his torsion balance should depend on gravity alone. 

 Hence he varied the apparatus in many ways, changing the 

 small balls subject to attraction, changing the connecting 

 rod, and the means of suspension. He observed the effect 

 of artificial disturbances, such as the presence of visitors, 

 the occurrence of violent storms, &c., and as no real altera- 

 tion was produced in the results, he confidently attributed 

 them to gravity 6 . 



Newton would probably have discovered the mode of 

 constructing achromatic lenses, but for the unsuspected 

 effect of some sugar of lead which he is supposed to have 

 dissolved in the water of a prism. He tried, by means of 

 a glass prism combined with a water prism, to produce 

 dispersion of light without refraction, and if he had 

 succeeded there would have been an obvious mode of 

 producing refraction without dispersion. His failure is 

 supposed to be due to his adding lead acetate to the water 

 for the purpose of increasing its refractive power, the lead 

 having a high dispersive power which frustrated his pur- 

 pose^ Judging from Newton's remarks, in the ' Philo- 

 sophical Transactions/ it would appear as if he had not, 

 without many unsuccessful trials, despaired of the con- 

 struction of achromatic glasses . 



The Academicians of Cimento, in their early and in- 

 genious experiments upon the vacuum, were often misled 

 by the mechanical imperfections of their apparatus. They 

 concluded that the air had nothing to do with the pro- 



e Baily, ' Memoirs of the Royal Astronomical Society,' vol. xiv. pp. 

 29-30. 



f Grant's ' History of Physical Astronomy/ p. 531. 



g 'Philosophical Transactions,' abridged by Lowthorp, 4th edition, 

 vol. i. p. 202. 



