238 Captain Abney and Major-General Festing. [Mar. 4, 



Palmer, Henry Spencer, Colonel 

 RE. 



Pickard- Cambridge, Rev. Octa- 



vius, M.A. 

 Poynting, Prof. John Henry, 



B.Sc. 



Pritchard, Urban, M.D. 

 Pye-Smith, Philip H., M.D. 

 Ramsay, Prof essor William, Ph.D. 

 Rodwell, George P., F.R.A.S. 

 Rnssell, Henry Chamberlaine, 

 B.A. 



Sanders, Alfred, F.L.S. 

 Sedgwick, Adam, M.A. 

 Snelus, George James, F.C.S. 

 Sollas, Professor William Johnson, 

 D.Sc. 



Stevenson, Thomas, M.D. 

 Tate, Professor Ralph, F.G.S. 

 Teale, Thomas Pridgin, P.R.C.S. 

 Tenison- Woods, Rev. Julian E., 

 M.A 



Tidy, Prof. Charles Meymott, M.B. 

 Tonge, Morris, M.D. 

 Topley, William, F.G-.S. 

 Unwin, Prof. W. Cawthorne, B.Sc. 

 Warington, Robert, F.C.S. 

 Wharton, William James Lloyd, 



Captain R.N". 

 Whitaker, William, B.A. 

 White, William Henry. 

 Wilde, Henry. 



Wright, Professor Edward Per- 

 ceval, M.A. 



The Bakerian Lecture was then delivered as follows : — 



I. The Bakerian Lecture. — "Colour Photometry." By 

 Captain Abney, R.E., F.R.S., and Major-General FESTING, 

 R.E. 



(Abstract.) 



One of the authors of this paper has already communicated to the 

 Physical Society of London (" Phil. Mag.," 1885) a method by which 

 a patch of monochromatic light could be thrown on a screen. This 

 formed the starting point of the present investigation, which was to 

 ascertain whether it was practicable to compare with each other the 

 intensity of lights of different colours. 



The authors describe various plans they adopted to effect this 

 purpose, and finally found that by placing a rod in front of the patch 

 of monochromatic light, and of a candle by casting another shadow, the 

 intensities of the two lights could be compared by what they term an 

 oscillation method. It is known that on each side of the yellow of 

 the spectrum the luminosity more or less rapidly decreases. By 

 placing a candle at such a distance from the screen that the luminosity 

 of the two shadows appears as approximately equal, it is easy to oscil- 

 late the card carrying the slit through which the monochromatic rays 

 of the spectrum pass. (The slit is in the focus of the lens which helps 

 to form the spectrum.) The shadow of the rod cast by the candle can 

 thus be made to appear alternately " too light " or " too dark " in com- 

 parison with the shadow of the rod cast by the parts of the spectrum 

 falling on the screen. By a rapid oscillation the position of equality 



