608 THE PKINCIPLES OF SCIENCE. [CHAP. 



branch of physical science has usually been developed from 

 the attention forcibly drawn to some singular substance. 

 Just as the loadstone disclosed magnetism and amber 

 frictional electricity, so did Iceland spar show the existence 

 of double refraction, and sulphate of quinine the pheno- 

 menon of fluorescence. "When one such startling instance 

 has drawn the attention of the scientific world, numerous 

 less remarkable cases of the phenomenon will be detected, 

 and it will probably prove that the property in question is 

 actually universal to all matter. Nevertheless, the extreme 

 instances retain their interest, partly in a historical point 

 of view, partly because they furnish the most convenient 

 substances for experiment. 



Francis Bacon was fully aware of the value of such 

 examples, which he called Ostensive Instances or Light- 

 giving, Free and Predominant Instances. " They are those," 

 he says, 1 "which show the nature under investigation 

 naked, in an exalted condition, or in the highest degree 

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

 strength predominating over and suppressing them." 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. It would not be easy to distinguish clearly 

 between these ostensive instances and those which he calls 

 Instantiav Monodicae, or Irreyulares, or Hderoclitae, 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. 2 



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 (p. 432). 

 Dollond, having noticed this peculiar dispersive power in 

 lenses made of flint glass, employed them to produce an 

 achromatic arrangement. The element strontium presents 

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

 remarkably low dispersive power; but I am not aware 

 that this property has yet been turned to account. 



1 Noijum Organum, bk. ii Aphorisms, 24, 25. 2 Ibid. Aph. 28. 



