Prof. S. P. Langley on Energy and Visio7i. 19 



whose transmission for different kinds of liglit was first pho- 

 tometrically measured and found to be 



For violet light (A,=0''-40) transmission 0-000210, 

 „ green „ (X=0''-55) „ 0-000655, 



„ red „ (X=0^'-65) „ 0-002350. 



The photometer-wheel was next interposed, its aperture 

 being sometimes reduced until only 2 per cent, of the light 

 received passed through it. 



The slit was at first kept as near the standard width of 

 0-1 mm. as possible ; but it was afterwards deemed best to 

 secure the final adjustment for the minimum visihile at the slit, 

 as it was evident on trial that the inaccuracy due to the vary- 

 ing 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, 0-00015 sq. 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 sq. cm., when fully expanded, or the same as that of the 

 foreshortened disk of figures previously employed. The size 

 of the light spot at the standard distance beyond slit Sg, when 

 the minute aperture is placed over the coilimating-lens, is 

 reduced so that about two thirds of the light enters the eye 

 placed 1 metre behind the 1 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 ininimum visibile, 

 definino; this to be not the smallest lio-ht 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. V. 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 liglit in 1 millim. of the standard 

 spectrum, whose length from A to H was 86 millim. 



C2 



