376 Proceedings of the Royal Institution, 



without knowing- what Bouguer had done, there can he' no aouht, as 

 he declares he had not even heard of Bouguer's work till many years 

 after liis discovery of the method of shadows, when Laplace put it 

 into his hands, and shewed him the method distinctly developed in 

 the writings of Bouguer. ,^ 



Mr. Ritchie then examined the principle proposed hy Bishq^p 

 Watson, and afterwards more fully developed by Professor LesUe ; 

 namely, the mode of ascertaining the relative intensities of dif- 

 ferent lights, by its absorption and conversion into heat. The 

 lecturer clearly demonstrated that this method could never be em- 

 ployed with even an approach to accuracy, when the colours of the 

 lights were different, as in that case the heat absorbed is by no means 

 -proportionate to the illuminating powers of the two lights. Mr. 

 Ritchie then described his exceeding delicate photometer of equality, 

 founded on the same principles with the differential photometer of 

 ; Professor Leslie, and shewed that when the colours of the lights were 

 nearly the same, the instrument gave a very good approximation to 

 the truth ; but that when the colours of the lights were different, the 

 same objections applied to it which he had made against the use of 

 Leslie's photometer. He then described the construction of his other 

 photometer, founded on the principle discoveredby Bouguer, viz., that 

 the eye can detect a very small difference between two discs of white 

 paper, placed in juxtaposition, when they are nearly equally illumi- 

 nated. This instrument consists of a square box, open at both ends, 

 and containing two mirrors, placed at right angles to each other. 

 The lid of the box has a rectangular opening, covered with a slip of 

 fine white paper. The instrument is pl^iced in a line between the 

 lights to be compared, and then moved towards the one or other, 

 till the discs on each side of the line which separates the two mirrors 

 l)e equally illuminated, and the squares of the distances of the lights 

 from the centre of the instrument will give their relative intensities. 

 In the course of the lecture he shewed the application of the instru- 

 ipent to determine the relative intensities of the brilliant hght afforded 

 by a small ball of lime ignited by the flame of oxygen and hydrogen 

 ga^es, so importantly applied by Mr. Drummond, and that of phos- 

 phorus burning in oxygen gas. He stated that, from accurate experi- 

 ments which he had made with Mr. Drummohct ^t Trinity-house,' uie 



