478 M. A. Claudet on the Dynactinometer, an Instrument 



even necessarily a function of the distance from the axis. As 

 this is a singular case of motion requiring peculiar considerations, 

 and as the motion of translation parallel to the axis is arhitraiy 

 and independent of the motion about the axis, I do not admit 

 that this one instance invahdates the general proposition that 

 udx + vdy + wdz is iutegrable by a factor. 



Cambridge Observatory, 

 May 12, 1851. 



LXVIII. Description of the Dynactinometer, an Instrument for 

 measuring the Intensity of the Photogenic Rays and comparing 

 the Power of Object-Glasses ; with Observations on the differ- 

 ence between the Visual and Photogenic Foci, and their constant 

 variation. By A. F. J. Claudet*. 

 [With a Plate.] 



THE various processes of photography are dependent on the 

 same acting principle ; and this principle, one of the ema- 

 nations of all luminous bodies, is the agent of a chemical action 

 by which certain compoimds undergo some modifications and 

 acquire new properties. It has received the name of actinism, 

 to distinguish it from light, because it can manifest itself in an 

 insulate state, and without reference to the degree of intensity 

 of light. Still the actinic rays are subject to the same laws of 

 transmission, reflexion, refraction and polarization. 



When a camera obscura is placed before an object impinged 

 upon by the luminous rays, these rays are refracted by the lens, 

 and converge to the focus of the camera, forming there an image 

 exactly similar to that represented upon the retina. 



But as the luminous rays are generally accompanied with ac- 

 tinic rays, these last form another image, which, although not 

 apparent to our senses, still will subsequently manifest its exist- 

 ence when we substitute for the screen of the camera a sm-face 

 coated with certain chemical compounds, which, by the modifi- 

 cation they experience under the action of the actinic rays, pro- 

 duce an image differing from the visual image in two important 

 respects, viz. the visual image does not last longer than the re- 

 flexion, and being produced by all the rays reflected from the 

 object, it is composed of the natural coloiu's, while the actinic 

 image, when once developed, continues permanent ; but being 

 entirely produced by a uniform action of the actinic rays alone, 

 the lights of the pictui-e can be expressed only by one tint and 



* Communicated by the Author, having been read before the British 

 Association the 7th of August 1850, in the Section of Mathematical and 

 Physical Sciences. 



