Luminous Intensity of Light, 



359 



liminaiy researches and discoveries are yet to be made, before a photometer 

 analogous to a thermometer in fixity of standard and facility of observa- 

 tion could be devised, the realization of an absolute light-measuring method 

 appears somewhat distant. The path to be pursued towards the attain- 

 ment of this desirable object appears to be indicated in the observations 

 which from time to time have been made by M. Becquerel, Sir John 

 Herschel, R. Hunt, and others, on the chemical action of the solar rays, 

 and the production thereby of a galvanic current, capable of measurement 

 on a delicate galvanometer, by appropriate arrangements of chemical baths 

 and metallic plates connected with the ends of the galvanometer wires. 



Many so-called photometers have been devised, by which the chemical 

 action of the rays at the most refrangible end of the spectrum have been 

 measured, and the chemical intensity of light tabulated by appropriate 

 methods ; and within the last few years Professors Bunsen and Roscoe 

 have contrived a perfect chemical photometer, based upon the action of 

 the chemical rays of light on a gaseous mixture of chlorine and hydrogen, 

 causing them to combine with formation of hydrochloric acid. 



But the measurement of the chemical action of a beam of light is as 

 distinct from photometry proper as is the thermometric registration of the 

 heat-rays constituting the other end of the spectrum. What we want is a 

 method of measuring the intensity of those rays which are situated at the 

 intermediate parts of the spectrum, and produce in the eye the sensation 

 of light and colour ; and, as previously suggested, there is a reasonable 

 presumption that further researches may place us in possession of a pho- 

 tometric method based upon the chemical action of the luminous rays of 

 light. 



The rays which affect an ordinary photographic sensitive surface are so 

 constantly spoken of and thought about as the ultra-violet invisible rays, 

 that it is apt to be forgotten that some of the highly luminous rays of light 

 are capable of exerting chemical action. Fifteen years ago* the writer was 

 engaged in some investigations on the chemical action of light, and he suc- 

 ceeded in producing all the ordinary phenomena of photography, even to 

 the production of good photographs in the camera, by purely luminous 

 rays of light free from any admixture with the violet and invisible rays. 

 When the solar spectrum (of sufficient purity to show the principal fixed 

 lines) is projected for a few seconds on to a sensitive film of iodide of silver, 

 and the latent image then developed, the action is seen to extend from 

 about the fixed line G to a considerable distance into the ultra-violet invisible 

 rays. When the same experiment was repeated with a sensitive surface of 

 bromide of silver instead of iodide of silver, the result of the development of 

 the latent image showed that, in this case, the action commenced at about 

 the fixed line b, and extended, as in the case of the iodide of silver, far 

 beyond the violet. A transparent cell, with parallel glass sides one inch 

 across, was filled with a solution of twenty-five parts of sulphate of quinine 

 * The Journal of the Photographic Society, vol. i. p. 98. 



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