260 THE PRINCIPLES OF SCIENCE. 



of power ; freed from impediments, or at least by its 

 strength, predominating over and suppressing them 1 .' He 

 mentions quicksilver as an ostensive instance of weight or 

 density, thinking it not much less dense than gold, and 

 more remarkable than gold as joining density to liquidity. 

 The magnet is mentioned as an ostensive instance of 

 attraction k . It would not be very easy to distinguish 

 clearly between these ostensive instances and those which 

 he calls Instantiate Monodicae, or Irregulares, or Hetero- 

 clitae, under which he places whatever is extravagant in 

 its properties or magnitude, or exhibits least similarity 

 to other things, such as the sun and moon among the 

 heavenly bodies, the elephant among animals, the letter 

 s among letters, or the magnet among stones ^ 



In optical science great use has been made of the high 

 dispersive power of the transparent compounds of lead, 

 that is, the power of giving a long spectrum (vol. i. p. 32). 

 Dolland having noticed the peculiar dispersive power of 

 lenses made of flint-glass employed them to produce an 

 achromatic arrangement. The element strontium presents 

 a contrast to lead in this respect, being characterized by a 

 remarkably low dispersive power ; but I am not aware 

 that this property has yet been turned to account. 



Compounds of lead have both a high dispersive and 

 a high refractive index, and in the latter respect they 

 proved very useful to Faraday. * Having spent much 

 labour in preparing various kinds of optical glass, Fara- 

 day happened to form a compound of lead, silica, and 

 boracic acid, now known as heavy glass, which possessed 

 an intensely high refracting power. Many years after- 

 wards in attempting to discover the action of magnetism 

 upon light he failed to detect any effect, as has been 



i ' Novum Organum,' bk. II. Aphorism 24. 

 k Ibid. Aphorism 25. 

 1 Ibid. Aphorism 28. 



