OF NATtiRAL PHILOSOPHY. 357 



devised by Messrs. Arago and Fresnel, for the 

 measurement of the refractive powers of transparent 

 media by means of the phenomenon of diffraction, 

 may be cited as affording a degree of precision 

 limited only by the wishes of the observer, and the 

 time and patience he is willing to devote to his 

 observation. And in respect of the direction of 

 observations to points from which real information 

 is to be obtained, and positive conclusions drawn, 

 the hygrometer of Daniell may be cited as an 

 elegant example of the introduction into general use 

 of an instrument substituting an indication founded 

 on strict principles for one perfectly arbitrary. 



(390.) In speculating on the future prospects of 

 physical science, we should not be justified in leav- 

 ing out of consideration the probability, or rather 

 certainty, of the occasional occurrence of those happy 

 accidents which have hud so powerful an influence 

 on the past; occasions, where a fortunate combin- 

 ation opportunely noticed may admit us in an in- 

 stant to the knowledge of principles of which no 

 suspicion might occur but for some such casual 

 notice. Boyle has entitled one of his essays thus re- 

 markably, " Of Man s great Ignorance of the Uses of 

 natural Things ; or that there is no one Thing in Nature 

 whereof the Uses to human Life are yet thoroughly 

 understood. * The whole history of the arts since 

 Boyle's time has been one continued comment on 

 this text ; and if we regard among the uses of the 

 works of nature, that, assuredly the noblest of all, 

 which leads us to a knowledge of the Author of 

 nature through the contemplation of the wonderful 



* Boyle's Works, folio, vol. Hi. Essay x. p. 185. 

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