6 REPORT— 1839. 



On an Apparatus for obtaining a numerical estimate of the Intensity of 

 Solar Light, at different periods of the day, and in different parts of 

 the globe. JBy Prof. Daubeny, F.R.S. 



The contrivance, by which it was proposed to effect this object, was 

 to consist of a sheet of photogenic paper, moderately sensible, rolled 

 round a cylinder, which, by means of machinery, would uncoil at a given 

 rate, so as to expose to the direct action of the solar rays, for the space 

 of an hour, a strip of the whole length of the sheet, and of about an 

 inch in diameter. Between the paper and the light was to be interposed 

 a vessel, with plane surfaces of glass at top and bottom, and in breadth 

 corresponding to that of the strip of paper presented. This vessel, 

 being wedge-shaped, was fitted to contain a body of fluid of gradually 

 increasing thickness, so that, if the latter were calculated to absorb light, 

 the amount of it intercepted would go on augmenting from one extre- 

 mity of the vessel to the other. Hence it was presumed, that any dis- 

 coloration that might arise from the action of light would proceed along 

 the surface of the paper to a greater or less extent, according as the 

 intensity of the sun's light was such as enabled it to penetrate through 

 a greater or lesser thickness of the fluid employed. In order to register 

 the results, nothing more would be required than to measure, each 

 evening, by means of a scale, how many degrees the discoloration had 

 extended along the surface of the paper which had been exposed to 

 light, during each successive hour of the preceding day. To render 

 the instrument self-registering, some contrivance for placing the paper 

 always in a similar position with reference to the sun, must of course 

 be superadded. The object of this contrivance differed from that aimed 

 at by Sir J. Herschel in his Actinometer, being intended as a measure 

 of the aggregate effect of the solar intensity at the period (be it long 

 or short) during which the paper was submitted to its influence; whereas 

 the Actinometer merely measures the intensity at the moment the ob- 

 servation is made. The interposition of an absorbing fluid has at least 

 this advantage, that it enables the observer to estimate the relative in- 

 tensity, by marking the point at which the paper ceases to be discolour- 

 ed, of which the eye is able to judge more exactly, than it could do, of 

 the relative darkness of shade produced on paper which had been ex- 

 posed without protection to light of different degrees of brilliancy. 



Notice respecting the Use of Mica in polarizing Light. 

 By Prof. Forbes, F.R.S. 



The author explained the method of preparing mica used by him, 

 since 1836, for the polarization of heat and light. The mica is exposed 

 for a short time to an intense heat in an open fire, by which the laminae 

 are so subdivided, that a pellicle of extreme thinness contains a suffi- 

 cient number of reflecting surfaces to polarize very completely the 

 light or heat transmitted through it at a certain degree of obliquity. 

 He next stated, that being struck by the resemblance to metallic lustre 

 which the mica acquires in this process, he had examme ( aiso in 1836) 



