60 Royal Society. 



" Experiments to determine the quantity of light reflected by plane 

 metallic specula under different angles of incidence, with a description 

 of the photometer made use of." By Richard Potter, Jun. Esq. Com- 

 municated by the President. 



Sir Isaac Newton has stated, that metallic specula, in common with 

 all other substances, reflect light most copiously when incident most 

 obliquely. Some experiments made by the author, with specula of 

 his own construction, having raised doubts in his mind as to the 

 accuracy of the prevailing opinion on this subject, which accords with 

 that of Newton and of Bouguer, he instituted a more exact inquiry 

 into the proportions of incident and reflected light from specula at 

 various angles of incidence. He used for this purpose a photometer 

 resembling that of Bouguer, and consisting of an upright screen, with 

 a square aperture, across which a piece of thin tissue-paper was ex- 

 tended, destined to receive on one compartment the reflected light 

 from one lamp, and on another compartment the direct light from 

 another lamp, employed as a standard of comparison. By adjusting 

 the respective distances of the lamps, the lights on the paper were 

 rendered sensibly equal in point of intensity, — the equality being 

 judged of by the eye, viewing them from the other side. The mea- 

 surements were taken alternately, first one of the direct, and then 

 one of the reflected, lights, until a sufficient number of uniform re- 

 sults were obtained. The author, after taking every precaution that 

 occurred to him for ensuring accuracy, invariably found that the pro- 

 portion of light reflected by metallic surfaces, instead of increasing, di- 

 minished in pretty regular gradation, as the angle of incidence was aug- 

 mented. Thus, in the first experiment, when the angle of incidence was 

 20°, the proportion of the reflected to the incident light was as 69*45 

 to 1 00, at 40° it was G6/9, and at 60° it was reduced to G4-9 1 . Some 

 irregularities occurred in the series of results deduced from different 

 sets of experiments, arising partly from the variableness of the light 

 given out by the lamps, and partly from the difficulty of preserving 

 the metallic surface in the highest state of lustre which it has when 

 newly polished. The author combats the opinion, that the quantities 

 of light which metals are capable of reflecting when polished are in 

 the ratio of their densities ; and finds, that in those metals which 

 were the subjects of his experiments, the quantities of light absorbed, 

 or lost by reflection, at incidences nearly perpendicular, are almost 

 exactly in the ratio of their specific heats. 



" On the theoretical investigation of the velocity of sound, as cor- 

 rected from M. Dulong's recent experiments, compared with the results 

 of the observations of Drs. Moll and Van Beck." By Dr. Simons, as- 

 sistant at the observatory of Utrecht. 



Laplace has demonstrated, that Sir Isaac Newton's formula for ob- 

 taining the velocity of sound, requires, in order to render it correct, 

 that it be multiplied by a certain co-efficient, depending on the ratio 

 between the specific heats of atmospheric air under a constant pressure 

 and under a constant volume. Laplace has endeavoured to deduce 

 this co-efficient, first from the experiment of MM. de la Roche and 

 Beiard ; secondly, from those of MM. Clement and Desormes - } and 

 lately from the more accurate investigations of MM. Gay-Lussac and 



Welter. 



