3S0 Mr. A. Claudet's Description of the Photographometer. 



ecliptic, having a triangular opening and turning upon its 

 axis, following the sun's path, clock-work movement being 

 used for the purpose. 



The photogenic paper is rolled upon another cylinder placed 

 in the interior of the first, and turning in a contrary direction. 

 The sun's rays, passing through the triangular opening, fall 

 successively upon all that portion of the paper exposed by the 

 rotation of the exterior cylinder. 



The photogenic effect produced upon the paper is in direct 

 proportion to the intensity of the light present during the 

 motion of the triangular opening, and also in direct proportion 

 to the length of the lines parallel to the base of the triangle at 

 every part of its opening; so that as the intensity of the light 

 increases or diminishes, the commencement of the photogenic 

 effect is indicated at a point more or less distant from the 

 base. At the end of the day, on removing the paper we find 

 a long figure, the height of the ordi nates of which is the mea- 

 sure of the intensity of the chemical action of the light. 



Mr. Hunt described his actinograph at the meeting of the 

 British Association at Cambridge in June 1845. 



Sir John Herschel in 1840 constructed an instrument for 

 the same purpose. He places photogenic paper under an 

 immoveable disc, having a narrow opening in the direction of 

 its radius for the passage of the sun's light reflected by a 

 heliostat: this light falls on the paper fixed upon another disc, 

 turning on its axis by clock-work. 



At the end of the day every part of the photogenic paper has 

 passed this opening, and the effect produced shows theintensity 

 of the light during the various hours from sunrise to sunset. 



All these several ingenious contrivances had no other object 

 than that of measuring the photogenic intensity of the direct 

 rays of the sun during the different hours of the day. Besides, 

 they were applicable only to photography on paper ; and the 

 results were uncertain, because the paper does not long pre- 

 serve the same degree of sensitiveness : then the heliostats 

 and clock-work movements rendered the apparatus compli- 

 cated, expensive and difficult to use. 



It may then be asserted, that up to the present time there 

 does not exist an instrument for indicating to the photogra- 

 pher the intensity of the chemical rays, and at the same time 

 the sensitiveness of his preparation. 



I have endeavoured to supply this want, and offer the result 

 of my labours in publishing an instrument which, I believe, 

 fulfils all the conditions necessary to render it useful, as it 

 measures at the same time the intensity of the chemical rays 

 and the sensitiveness of the preparation. I have called it a 



