46 



PHYSICAL AND MATHEMATICAL SECTION. 



January 15tli, 1884. 



Alfred Brothers, F.R.A.S., in the Chair. 



" Note on Bouguer's Optical Essay on the Gradation of 

 Light," by James Bottomley, B.A., D.Sc, F.C.S. 



In several papers on colorimetry which I have read before 

 this Society, I have frequently had occasion to refer to the" 

 hypothesis, that as the length of an absorbing medium in- 

 creased according to the terms of an arithmetical progression, 

 the intensity of the light diminished according to the terms of 

 a geometrical progression. 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 enquiry, there has come into my hands 

 a small treatise entitled Essai d'Optique sur la gradation de 

 la lumiere, par M. Bouguer, professeur royal en Hydro- 

 graphie. Paris, 1729. From this work it appears that the 

 honour of having first enounced the hypothesis belongs to 

 Bouguer. In many otherwise excellent treatises on Physics 

 and Optics the subject of the absorption of light is either 

 neglected or scantily treated, and the claims of Bouguer 

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

 assuredly claim herein a position correlative with that as- 

 signed to Snell, or Huygens, 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 enquiries, and with regard to his own work he 

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



