62 Capt. Abney on the Measurement of the Luminosity and 



diameter '02 inch, wound in a screw-thread of pitch '025 

 inch, cut in our Whitworth lathe on a brass cylinder of dia- 

 meter 22 inches, the number of turns of wire being 185. For 

 such a coil it seemed probable that Lord Rayleigh's formula, 

 which is a first approximation, might not give a result of 

 sufficient accuracy for the purpose in view ; and when Lord 

 Rayleigh, with whom I had the privilege of speaking on the 

 matter last summer, expressed this opinion, it became clear 

 that a formula embodying a closer approximation would have 

 to be worked out. This might be done by considering the 

 helix as a current-sheet, and proceeding to a second approxi- 

 mation on this hypothesis. But it seemed to me preferable 

 to make a direct attempt at integration for the case of helix and 

 circle, though I hardly anticipated that I should arrive in the 

 result at a formula of such simplicity as that given above. 



It is interesting to observe that, by Lord Rayleigh's for- 

 mula, the coefficient of mutual induction of the circle and 

 helix taken above for purposes of calculation comes out to be 



wx 53-317. 



The difference between the two results is about one tenth per 

 cent. 



University College of South Wales 

 and Monmouthshire, Oardilij 

 November 8, 1888. 



VII. On th& Measurement of the Luminosity and Intensity 

 of Light reflected from Coloured Surfaces. By Capt. W. 

 DE W. Abney, C.B., R.E., F.R.S* 



IN a communicationf to the Royal Society General Festing 

 and myself have shown how to compare the light of the 

 different parts of the spectrum reflected from a white surface 

 with that reflected from a coloured surface, and we gave the 

 results of measurements of various colours, and from these 

 constructed their luminosity curves by means of the lumi- 

 nosity curve of the spectrum of white light, which we had 

 ascertained from our previous researches on Colour Photo- 

 metry (Bakerian Lecture 1886). From the areas of these 

 curves we deduced the total luminosity of these colours, com- 

 pared with that of a white surface. Certain colours were 

 combined b}^ means of rotating sectors to form a grey, and 

 this was matched with the grey formed by rotating sectors 

 of black and white. By noting the angular value of each 



* Communicated hy the Physical Society : read November 24, 1888. 

 t " Colour Photometry," Part II., Phil. Trans. Part A, 1888. 



