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LXX V. A Reply to the Observations of 5. H. Wheeler, Esq, 

 on the Method of computing the Results of Experiments "with 

 the Comparative Photometer. By Richard Potter, Esq.y 

 B.A.I' 

 IN the Number of the Philosophical Magazine and Journal 

 ■*■ of Science for December 1 834, (vol. v. p. 439.) is a paper by 

 J. H. Wheeler, Esq., in which he gives the results he has 

 obtained on repeating certain of my photometrical experi- 

 ments published in previous Numbers, and his calculations 

 founded upon them. 



It is satisfactory to find that my experimental results have 

 been verified in the most complete manner by Mr. Wheeler, 

 who cannot for a moment, from the tenour of his paper, be 

 suspected of having his experiments influenced by any pre- 

 possession against the undulatory theory of light. I regret 

 however that we do not agree equally well as to the mode to 

 be pursued in calculating the quantity of light transmitted or 

 reflected, from the data furnished by the experiments. Mr. 

 Wheeler speaks of the possibility of his continuing his re- 

 searches, which induced me to believe that he would finally 

 perceive the true theory of the comparative photometer, and 

 in some later essay would give a correction of his former 

 views; and from the polite tone of his paper, I should have 

 much wished that it had come from his own pen rather than 

 mine. Another reason for my not repl}^ing earlier to Mr. 

 Wheeler was, that I had formed a resolution to avoid all 

 scientific controversies whilst an undergraduate of this Uni- 

 versity. 



The preliminary formulae which Mr. Wheeler investigates 

 are perfectly correct for the illumination received on a screen 

 from any small luminous surfaces inclined at any angles to 

 that screen ; but the comparative photometer acts on a quite 

 different principle. When we compare the reflective powers 

 of two substances, for example, those exhibited by plane po- 

 lished surfaces of diamond and glass, we view the images 

 which these surfaces give of small portions of a long and nar- 

 row parallelogram of pasteboard, which is bent round into a 

 semicircle, and equally illuminated in every part, the reflecting 

 surfaces being placed at the centre of the semicircle. Now, 

 all the parts of the pasteboard being, by supposition, equally 

 illuminated, the images of the parts of it seen by the eye re- 

 flected in the surfaces of the glass and diamond, would ap- 

 pear equally bright if the same proportion of the incident rays 

 were reflected by each. We do not regard the reflector, but 



• Communicated by the Author. 



