92 PROCEEDINGS OF THE AMERICAN ACADEMY 



mentum; according to the theoiy of Leibnitz and his fol- 

 lowers, it is the mass multiplied by the square of the velocity. 

 The philosophers who have maintained the first theory are 

 chiefly English ; those who favor the latter theory are 

 chiefly Continental. The object of the communication was 

 to render intelligible, by a simple illustration, the truth of 

 the second theory. 



Dr. A. A. Hayes exhibited and described a modification of 

 the Photometer invented by Ritchie, by which the illuminat- 

 ing power of two flames can be directly compared. 



He alluded to the diff'erent methods by which the attain- 

 ment of an accurate measurement had been sought for, by in- 

 tercepting light and comparing shadows, and pointed out the 

 objections to them ; concluding by expressing his opinion that 

 Bunsen's mode, in its adaptation by Mr. King, with the im- 

 provements of Mr. Lewis Thompson, gives the nearest ap- 

 proximation to correctness which has been attained. 



The instrument exhibited, in the arrangement of the two 

 mirrors and admission of light from the two flames, was es- 

 sentially Ritchie's. But the modification which renders it a 

 more accurate indicator, and more generally useful, consists 

 in reversing the position of the mirror-plates, and removing 

 the oiled paper, so that the two beams of reflected light are 

 projected downward in a small darkened chamber upon a 

 printed page. Two rectangles of light, side by side, are thus 

 made to illuminate a page, the printed lines on which pass 

 across the chamber and have the same words occurring within 

 the lighted space from each flame. The page is viewed 

 through a partly opened leaf in front, and being at a conven- 

 ient distance from the eye, a slight inequality in the light on 

 either side is readily seen. 



In making the experimental comparisons, the centre of one 

 mirror in the instrument was placed at one hundred and twenty 

 inches from a gas flame by moving the light pedestal support- 

 ing it, and on which it slides. A spermaceti candle burning 



