118 Sir W. de W. Almey. On the Estimation of the 



" On the Estimation of the Luminosity of Coloured Surfaces used 

 for Colour Discs." By Sir WILLIAM DE W. ABNEY, K.C.B., 

 F.RS. Received May 5, Read May 31, 1900. 



When a source of light is small, such as the points of an arc light, a 

 candle, or lamp, it is comparatively easy to find the luminosity of any 

 coloured surface which is illuminated by it, using the method which has 

 been described in " Colour Photometry, Part II " ;* but when the source 

 of light is a large surface, such as the sky, the method therein described 

 is much more difficult to apply. Quite recently, when examining the 

 question of providing suitable screens for producing the negatives 

 required for three-colour photographic prints, it became necessary to 

 devise a plan by which rings of different colours could be made of equal 

 luminosity in ordinary daylight by rotating them with the proper 

 proportions of black. The rings were concentric and rotated as a disc, 



Fia. 1. 



S is the nut of the spindle. 



Vis a yiolet disc (methyl violet). 



S is a portion of a blue ring (French ultramarine). 



R red ring (Yenniliou). 



G green ring (emerald green). 



1* yellow ring (chrome yellow). 



W white ring. 



see fig. 1 , and the difficulty encountered was to ascertain what amount 

 of black ought to form part of each ring. 



In " Colour Photometry, Part III,"t it was shown that only one ray 



* Abney and Testing, Phil. Trans.,' A, 1888. 

 t Abney and Festing, ' Phil. Trans.,' A, 1892. 



