212 INTENSITY OF LIGHT THROUGH AN ABSORBING MEDIUM. 



medium increased according to the terms of an arithmetical 

 ratio, the intensity of the light diminishes according to 

 the terms of a geometrical ratio. I had not then been 

 able to trace this hypothesis back farther than the writings 

 of Sir John Herschel, but had some grounds for supposing 

 that it might have been given earlier, and more especially 

 by Bouguer. Lately, after much inquiry, there has come 

 into my hands a small treatise, entitled ' Essai d'Optique 

 sur la Gradation de la Lumiere, par M. Bouguer, prof'es- 

 seur royal en Hydrographie/ Paris, 1729. From this 

 work it appears that the honour of having first announced 

 the hypothesis belongs to Bouguer. 



In many otherwise excellent treatises on Physics and 

 Optics the subject of absorption of light is either 

 neglected or scantily treated, and the claims of Bouguer 

 seem to have nearly passed ont of recognition ; yet he may 

 assuredly claim herein a position correlative with that 

 assigned to Snell, or Huyghens, or Newton in those depart- 

 ments of optics of which they laid the foundations. The 

 treatise contains no experimental verification of the hypo- 

 thesis, nor any suggestions for carrying out such experi- 

 ments. He was aware that the subject afforded a vast 

 field for future inquiries, and with regard to his own work 

 he modestly states, in the preface, ""C'est vrai que mes 

 recherches sont poussees si peu loin qu'elles laissent encore 

 un vaste champ a tous ceux qui voudront perfectionner 

 cette matiere. Mais ne scait-on pas que les Arts les plus 

 simples ont eu leurs differens ages, et que ce seroit comme 

 etoufer dans le berceau les decouvertes qu'on peut faire 

 dans la suite, que de mepriser toutes les premieres tenta- 

 tives, sous pretexte que ce ne sont encore que de foibles 

 commencemens ? " 



In the last section he has also considered the intensity 

 of light which has passed through a medium which is not 



