278 



Lord Rayleigh. On the Intensity of [Nov. 18, 



Description of Apparatus. 



The light is admitted into the room through a pane of finely- 

 ground glass fitted into the shutter. All other light is carefully 

 excluded, and the walls and ceiling are blackened — an almost indis- 

 pensable provision. The ground glass carefully cleaned* is illuminated 

 not only by the direct light of the sky, but also by light from above 

 reflected at a large mirror. The reception of light through a large 

 angle not only favours the aggregate brightness and tends to moderate 

 the changes due to passing clouds, but it makes the uniformity of 

 the field more independent of the evenness with which the glass is 

 ground. Under these circumstances, and when there is no sunshine, 

 direct or reflected, falling upon the ground glass, the latter may be 

 looked upon as a tolerably uniform source of diffused light. This 

 uniformity, however, is not relied upon ; but the arrangements are so 

 made that the parts of the field compared are contiguous or identical, 

 and are seen by rays which leave at the same angle. It will be con- 

 venient first to describe generally the course of the light, and after- 

 wards the manner in which the adjustments were effected. 



Proceeding from the ground glass (A), fig. 1, the light falls upon 

 the transparent plates, B, B', at which nearly equal parts are reflected. 

 These plates are of worked glass about 6 inches by 4 inches, and are 

 placed at the polarising angle. By this means are obtained two 

 beams of polarised light of nearly equal, and of constant relative, 

 brightness. The light transmitted by both plates is stopped at a 

 screen, and takes no further part. 



On the right hand side the light reflected at B is again reflected by 

 a mirror of worked glass, silvered behind, at C, and assumes the 

 direction CDF. D and F are alternative positions of the same 

 mirror (also of worked glass, silvered behind). When the glass 

 under test E is in use, the shifting mirror is in the position D, and 

 the light follows the course CDEFH. In the contrary case, there is 

 one reflection instead of two, and the ray takes the finally identical 

 course CDFH. At H this central ray is reflected at the extreme edge 

 of a speculum of silver-on-glass in the direction HI, to a small 

 observing telescope, which is focussed upon this edge. 



By adjustments that will presently be explained, it is secured that 

 the reflections at D and F shall take place under the same angle, and 

 therefore with the same (moderate) loss of light ; but when E is in 

 use the brightness is diminished some twenty times. To compensate 

 this in the other position the disk Gr is introduced. It consists 

 of a blackened disk of tin, from which (along a sufficient length 

 measured radially) a sector is cut out, • so that when the disk is 



* Strong sulphuric acid is an excellent detergent for this purpose. 



