S. P. Langley — Energy and Vision. 375 



which are at the same time capable of even approximate nu- 

 merical estimation. Out of numerous plans tried, the following 

 has given the most reliable result. 



In front of the first slit, in the path of the rajs from the 

 siderostat, was placed a plate of glass very lightly smoked whose 

 transmission for different kinds of light was first photometri- 

 cally measured and found to be 



For violet light (*=0^-40) transmission 0*000210 

 " green " (A=0^55) " 0*000655 



" red " (*=0^*C5) " 0*002350 



The photometer wheel was next interposed, its aperture be- 

 ing sometimes reduced until only 2 per cent of the light re- 

 ceived passed through it. 



The slit was at first kept as near the standard width of 0*l mm 

 as possible : but it was afterwards deemed best to secure the 

 final adjustment for the minimum visibile at the slit, as it was 

 evident on trial that the inaccuracy due to the varying loss by 

 diffraction was small, compared with the inevitable uncertainty 

 of the observer himself. 



Finally, the larger part of the necessary reduction was 

 secured by reducing the aperture of the collimating lens by 

 means of a metal plate pierced by a minute aperture whose 

 area, 00015 ,q cm , was 0*000003 of the fully illuminated area 

 of the lens. 



The aperture of the human eye, according to du Bois-Rey- 

 mond's photograph (see Nature, May 3, 1888, p. 15), is about 

 0-7 8q cm , when fully expanded, or the same as that of the fore- 

 shortened disk of figures previously employed. The size of 

 the light spot at the standard distance beyond slit s 3 , when the 

 minute aperture is placed over the collimating lens, is reduced 

 so that about two-thirds of the light enters the eye placed 1 

 meter behind the l mm slit on which the spectrum is formed. 



The following reductions of sunlight were needed in order 

 to give a light which approximated to the minimum visibile, 

 defining this to be, not the smallest light whose existence it is 

 possible to suspect, or even to be reasonably certain of, but a 

 light which is observed to vanish and reappear when silently 

 occulted and restored by an assistant without the observer's 

 knowledge. 



Referred to the standard spectrum employed in the previous 

 photometric work, the observer F. W. Y. found : 



Fraction of standard* violet light (A= 0^*40) required for certain 

 vision =0-00021 X 100 X 0*000003 = 0*000000,063. 



* By "standard" is here meant the light in l mm of the standard spectrum, 

 whose length from A to H was 86 mm . 



