Dr. J. B. Hay craft. Luminosity and Photometry. 49 



EH 12. In old brick-pit 90 yards S.S.W. of the Charlton road and close to path to 

 Maydencroft Farm. Level (of old surface) 271 feet. 



Feet. 



_ , f Yellow brickearth with small stones (9 feet already 

 Palaeolithic. \ , N v ui 

 L removed) 14^ 



Ancient J Yellow and white marl and silt (compare BH 8) 2 



alluvium. 1 Yellow brickearth and small chalk pebbles | 



r Chalky boulder clay 9 



G-lacial. < Loamy chalky gravel 2 



L G-raveliy sand « 8 



36 



BH 13. Old gravel pit in Jeeves' Yard, near Black Horse Lane. Level (of old 



surface) 242 feet. 



Feet. 



G-lacial \ Coarse gravel, becoming finer below. (Dug) 18 



I Finer gravel, watery below. (Bored) 10J- 



28£ 



BH 14 and trial -pit (see p. 44). 



*' Luminosity and Photometry." By John Berry Haycraft, 

 1LD., University College, Cardiff. Communicated by 

 Professor Schafer, F.R.S. Received February 9, — Read 

 March 4, 1897. 



The luminosity of the spectrum was determined by what may be 

 termed the method of the " minimal effective stimulus." Through 

 the partition separating two dark rooms from each other a hole was 

 drilled 1 mm. across ; this was covered with a piece of ground glass. 

 From a diffraction grating in room A spectral rays of ascertainable 

 wave-length were cast upon the ground glass. The observer in 

 room B was placed upon a graduated railway, and moved towards 

 or from the spot of light until he could just see it, and no more. 

 The distances at which various parts of the spectrum were just seen 

 were thus determined, and their relative potencies calculated by the 

 law of inverse squares. Under the above conditions — the eye being 

 in the condition of dark adaptation — the green near E is the most 

 potent, and the violet half of the spectrum is more potent than the 

 red. These results agree with those obtained by Captain Abney who 

 also worked with the dark adapted eye. 



The experiment was then repeated, the room B being white- 

 washed, and lit with gas light, and the eye of the observer, therefore, 

 in the condition of light adaptation. Under these conditions the 

 yellow is the most potent, and the red half of the spectrum more 

 potent than the violet. 



In the following experiment the eye was in the condition of 



