1897.] NEW YORK ACADEMY OF SCIENCES. 191 



and its source of light was then varied until the two surfaces ap- 

 peared to be of the same luminosity. The relative intensities of the 

 two lights were then calculated h}' the law of inverse squares. 



Lambert* made use of the principle of two shadows to 

 obtain two adjacent surfaces, one illuminated by a source A, the 

 other by a source B, to be compared with A. The distance of 

 the light B, from the screen was varied until the luminosities of 

 the two shadows appeared to be the same. The ratio of the 

 luminous intensities of the two lights could then be calculated 

 by the law of inverse squares. 



Rumfordf used the same principle as Lambert but made 

 man}^ improvements in its application. 



Potter J modified Bouger's photometer by replacing the 

 opaque white screens b}^ a single translucent one and putting the 

 lights to be compared behind the screen. 



Richie§ made still further changes in the Bouger photometer. 

 The lights to be compared were placed at opposite ends of a 

 table and two mirrors forming a wedge, the faces of which were 

 inclined at angles of forty-five degrees to the line connecting 

 the lamps, reflected the light to a translucent screen placed in 

 front of the wedge. The next modification in this form of pho- 

 tometer was to replace the mirrors by a wedge of an angle of 

 ninety degrees, covered with white paper. The two faces of 

 the wedge occupied the positions of the two mirrors and the trans- 

 lucent screen was dispensed with. 



In all the photometers above referred to, the dependence 

 upon what I will call the principal of equal luminosities of ad- 

 jacent surfaces is very evident. In nearly all the boundary be- 

 tween the two surfaces is quite distinct even when equality of 

 illumination is obtained. This detracts from the accuracy with 

 which slight differences of luminosity can be detected. To avoid 

 this source of error a number of devices have been resorted to, 

 all of which more or less conceal the use of the principle of 

 equal luminosities. A slight inspection of the photometer, how- 

 ever reveals it. 



Pernot || increased the accuracy of the Rumford photometer 

 by using a translucent screen and viewing the shadows from be- 

 hind it. Instead of judging directly concerning the equality 

 in luminosities of the two shadows, he brings a third source of 



light up from behind the screen and if both shadows disappear 



.. _ ^ •* I 



♦Photometria Sine de Mensura et Gradibus Luminis, Colorum et Umbrae. 1760. 



t Philosophical Transactions of the Royal Society of London. Vol. LXXXIV. 1794. 



i Edinburgh Journal of Science. New Series, III, page 284. 



§ Annals of Philosophy. Third Series. Vol. I. Page 174. 1826. 



llDinglersPolytech. Journal, CXIX. 1851. 



