124 Dr. S. P. Thompson on Photometry. 



divided block passes through the state of equality, from being 

 brighter on the one side to being brighter on the other, the 

 narrow dark line which divides the two luminous portions 

 appears itself to shift, the apparent displacement being away 

 from the more luminous side. Whether this optical effect 

 arises from irradiation, or from any other cause, it certainly 

 assists the eye in its judgment as to the position of balance. 



Some further observations with this device are still in 

 progress. 



III. The Electric- Arc Standard of Light. 



In 1878 experiments were made at Chatham by Abney, 

 Cardew, and others upon the electric arc, in the course of 

 which the practical invariability of the intrinsic illumination 

 of the crater- surface of the positive carbon of the arc was 

 established. Abney and Festing, in their researches on the 

 photometry of colour, have since that period used as a 

 standard of white light the light of the crater of the arc. As 

 pointed out by the writer some years ago, this invariability of 

 whiteness, which implies invariability of temperature, is 

 necessarily due to the constancy of the temperature of vola- 

 tilization of carbon. The introduction into the substance of 

 the carbons of any material having a lower temperature of 

 volatilization, or of any compound which has a temperature 

 of dissociation lower than that of the volatilization of carbon, 

 necessarily lowers the intrinsic brilliancy of the light. This 

 having once been realized, it seemed only natural to suggest 

 as a standard of light the light emitted from a given area of 

 the crater-surface of a pure carbon. This suggestion was 

 made by the author last year when writing from Rome some 

 remarks for the discussion of the paper of Mr. Trotter on the 

 light of the electric arc, read before the Institution of Elec- 

 trical Engineers. A similar suggestion was independently 

 made by Mr. Swinburne in the same discussion. For some 

 time past the author has been considering the experimental 

 methods for putting into practice the suggestion then made. 

 The only way to secure a constant effective area of crater 

 is to produce a much larger crater than is required, and 

 cover it by an opaque screen pierced with a suitable aperture 

 of standard dimensions. As this screen must be placed very 

 near the arc, it must be kept cool artificially by circulation of 

 water *. 



* M. A. Blondel has recently presented to the Societe de Physique 

 of Paris an apparatus denominated arc normale, in which is embodied 

 this method of carrying out the proposal of Mr. Swinburne and of the 

 author. 



