422 RECENT PROGRESS IN PHYSICS. 



l! 



It is to be regretted that we have no measures of the intensities of ■ 

 the galvanic light, when different metals are used instead of the 

 carbon points. 



Fizeau and Foucault have also made comparative experiments on the 

 intensity of the galvanic arc light, but from another point of view. — 

 (Ann. cie Chim. et de Phys. ser. Ill, T. XI, pp. 370; Pog. Ann., 

 LXIII, 463.) They did not compare the intensity of the light from \ 

 different sources, but its chemical effect. In this way they compared 1 

 the galvanic light with that of the sun, and of lime incandescent in i 

 detonating gas. The experiment was conducted in the following ;; 

 manner: An iodized silver plate was inserted in a camera obscura, in i 

 the place where the image of the sun or of the light, emanating from ] 

 the carbon or lime, was formed. After a short action of the light the :' 

 camera obscura v/as closed, and the position changed, so that another ■ 

 image of the object was shown upon the prepared silver plate beside : 

 the first one ; the exposure was somewhat longer than before ; for a i 

 third place still longer, &c. The plate was then put into the mercury ' 

 bath and examined, in order to find which one of the images became : 

 visible by the action of the mercurial vapor. In this way it was ascer- 

 tained how long the light had to act in order to produce that change ' 

 in the iodide of silver, which is necessary for the condensation of the 

 mercurial vapor. 



If all the other circumstances were entirely identical, the time re- 

 quired for the production of the Daguerrean image would be nearly ■ 

 inversely proportional to the chemical intensity of the corresponding 

 sources of light. 



But Fizeau and Foucault used for their experiments with the arti- 

 ficial light lens of shorter focus than for obtaining images of the sun ; 

 the aperture of the lens also was varied by means of diaphragms. 

 These circumstances have, therefore, to be taken into account. 



If the image is n times further from the lens, it will, caBteris 

 paribus, be n times greater in its linear diriiensions, and will, there- 

 fore, cover a surface n^ times as large, and consequently the in- 

 tensity of light at each point of the image will be n^ times less. 

 The chemical power of the source of light may, therefore, be con- 

 sidered proportional to the square of the distance of the image formed 

 from the lens. 



But it is also, as easily perceived, inversely proportional to the sur- 

 face of the opening of the lens, i. e., to the square of its radius, and 

 therefore 



T_ d^ 



when J denotes the chemical power of the source of light, d the dis- 

 tance of the image from the lens, r the radius of its opening, and t 

 the time required to produce a Daguerrean image. 



If we denote by « the angle which the radius of the aperture of the 

 lens subtends at the place of the image, then 



— = tang, a 

 a ° 



therefore J = r~^ 



t. tang, a 



