432 THE PRINCIPLES OF SCIENCE. [CHAP. 



It was in this cautious spirit that Baily acted in his 

 experiments on the density of the earth. The accuracy 

 of his results depended upon the elimination of all disturb- 

 ing influences, so that the oscillation of his torsion balance 

 should measure gravity alone. Hence he varied the appa- 

 ratus in many ways, changing the small balls subject to 

 attraction, changing the connecting rod, and the means of 

 suspension. He observed the effect of disturbances, such 

 as the presence of visitors, the occurrence of violent storms, 

 &c., and as no real alteration was produced in the res.ults, 

 he confidently attributed them to gravity. 1 



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 

 attributed 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 purpose. 2 

 Judging from Newton's remarks, in the Philosophical 

 Transactions, it would appear as if he had not, without 

 many unsuccessful trials, despaired of the construction of 

 achromatic glasses. 8 



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 produc- 

 tion of sounds, evidently because their vacuum was not 

 sufficiently perfect. Otto von Guericke fell into a like 

 mistake in the use of his newly-constructed air-pump, 

 doubtless from the unsuspected presence of air sufficiently 

 dense to convey the sound of the bell. 



It is hardly requisite to point out that the doctrine of 

 spontaneous generation is due to the unsuspected presence 



1 Baily, Memoirs of the Royal A37ronomical Society, vol. xiv. pp. 



29, 3- 



- Grant, History of Physical Astronomy, p. 531. 



3 Philosophical Transactions, abridged by Lowthorp, 4th edition, 

 vol. i. p. 202. 



