360 Prof. W. Thomson's Notes on Atmospheric Electricity. 



fg the system of rings in the quadrant is seen in right-circularly 

 (elliptically) polarized light ; if hf is covered, the rings in the 

 quadrant in red light are seen in left-circular (elliptical) light ; 

 by removing the slide, the cross whose arms are on one side of a 

 different colour than the other. 



The polarizing apparatus which I have described gives an 

 objective representation of the phenomena. It is simply neces- 

 sary to concentrate an intense light on the aperture of the 

 polarizing Nicol through an objective, and to remove the ocular 

 from the analysing nearer to the polarizing Nicol. The light 

 emerging from the analysing Nicol is caught on a white surface, 

 on which the rings are represented in corresponding size. I 

 have not applied this to the dichrooscope, for in fact the darkening 

 of the objective pictures is considerable when a deeply-coloured 

 glass is held before the eye which is examining it. 



The dichrooscope can be combined with Nuremberg's appa- 

 ratus in the following manner. For the composition of the 

 interference colours, a plate must be added parallel to the po- 

 larizing plate; the coloured glasses stand then one over the 

 other in the same vertical plane. For imitating the dichroitic 

 phenomena, the glass plates are placed so that these planes of 

 reflexion are at right angles to each other. On account of the 

 difficulty of the illumination, the apparatus can only be used 

 immediately in the neighbourhood of the window, which, on 

 account of the side light, greatly limits the intensity of the 

 colours of polarization. In this respect an apparatus is to be 

 preferred which, as it can be directed towards the source of light 

 like a telescope, can be used as well by day as by night in any 

 part of a room. When it is placed parallel to the earth's axis, 

 it can be used as a sun-clock even in twilight, according to a 

 process described in PoggendorfPs Annalen, vol. xxxv. p. 596, — 

 an application which I made {Maas und Messen, p. 62) in 1845, 

 ten years before these apparatus appeared under the special 

 name of Polar-clocks. 



Several of the dichrooscopes which I have described have been 

 very neatly made by Langhoff, the philosophical-instrument 

 maker. 



XLVIII. Notes on Atmospheric Electricity. 

 By Professor W. Thomson, F.R.S.* 



TWO water-dropping collectors for atmospheric electricity 

 were prepared, and placed, one at a window of the Natural 

 Philosophy Lecture-room, and the other at a window of the 



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

 Association, June 1860. • 



