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XXIV. On Solar Light, and on a Simple Photometer. By Mungo Ponton, Esq., 



F.R.S.E. 



(Read 4th March 1856.) 



In approaching the subject of solar light, the first point is to endeavour to 

 form an idea, not altogether indefinite, with respect to its quantity and intensity, 

 as compared with some familiar standard of artificial flame. With this view 

 were made, in the course of last summer, the observations now to be described. 



After several trials, it was found, that the most convenient mode of procedure 

 was first to compare a definite small surface, illuminated by solar light, with a 

 like surface illuminated by the mere light of the sky, and then to compare the 

 latter with a similar surface illuminated by the flame of a moderator lamp. 

 The light of the sky thus affords a middle term between the extreme lights of the 

 sun and the lamp, which are too diverse to be directly compared. 



The first difficulty to be overcome, was that arising from the difference of 

 colour between the flame of the lamp and the light of the sky. For this purpose, 

 it was found necessary to employ light of only one colour ; and blue light was 

 selected, as that which could be most easily obtained pure. 



Various methods having been tried, it was found that the blue rays could be 

 obtained in sufficient purity, by taking the common blue paper used by haber- 

 dashers for packing their light goods, and steeping it in a concentrated solution 

 of sulphate of copper, and then viewing this paper through common blue glass. 

 This glass, it is well known, transmits only the blue rays and the extreme red ; 

 but the blue paper absorbs the extreme red, and disperses only the blue and a 

 few yellow rays, which last are absorbed by the blue glass ; so that, by this com- 

 bination, only the blue rays reach the eye. In the light thus obtained, no rays 

 save the blue could be detected by prismatic analysis. 



To regulate with exactness the quantity of light admitted to the eye, a num- 

 ber of small slips of tea-lead were perforated with holes of various diameters, from 

 j^th down to xoo^h of an inch. These apertures were carefully made, the rugged 

 edges being removed, so as to present a clean circular outline. The diameter of 

 each hole was exactly measured under the microscope, with a power of a hun- 

 dred diameters. 



The next point was to secure the exclusion from the eye of all extraneous light. 

 For this purpose two pasteboard tubes were made, about eight inches long, and 

 one inch in diameter, and lined inside with dull black paper. These were placed 

 parallel to each other, and fastened together in such a manner that the centres 



VOL. XXI. PART III. 5 F 



