The new flicker photometry : thesis submitted ... for the degree of doctor of philosophy in the faculty of Pure Science, Columbia University.
- Tufts, F.L.
- Date:
- 1897
Licence: Public Domain Mark
Credit: The new flicker photometry : thesis submitted ... for the degree of doctor of philosophy in the faculty of Pure Science, Columbia University. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![[mar. 1, and its source of light was then varied until the two surfaces ap. j peared to be of the same luminosity. The relative intensities of the j two lights were then calculated by the law of inverse squares. i Lambert* made use of the principle of two shadows to 1 obtain two adjacent surfaces, one illuminated by a source A, the i 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 i by the law of inverse squares. 1 Rumford'j used the same principle as Lambert but made ; many improvements in its application. Potter J modified Bouger’s photometer by replacing the opaque white screens by 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 j 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 j 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 diflferences 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 * Photometria Sine de Mensura et Gradlbus Luminis, Colorum et Umbrse. 1760. + Philosophical Transactions of the Royal Society of London. Vol. LXXXIV. 1794. I Edinburgh Journal of Science. New Series, III, page 284. a Annals of Philosophy. Third Series. Vol. 1. Page 174. 1826. if Dlnglers Poly tech. Journal, CXIX. 1861.](https://iiif.wellcomecollection.org/image/b22469746_0006.jp2/full/800%2C/0/default.jpg)
