Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
175, 176.] 
§22. Duration of the Sensation of Light 
it gives out is distributed uniformly over the whole disc; then the 
luminosity will be diminished in the ratio of the area of the white 
sectors to that of the whole disc; and hence it will be H. Let r and 
R denote the distances of the sectored and pure white discs, respec¬ 
tively, from the source of light, when they both look equally bright; 
nw H H r2 nw 
360 r2 ~ R2 °r R2 ~ 360 ' 
Plateau’s measurements agree very well also with this law. 
The author has used another method too as follows. In the case 
of a disc covered with narrow black and white sectors, an apparently 
uniform distribution of the light from the white sectors can be pro¬ 
duced over the entire disc by viewing it through a convex lens that 
prevents accommodation. If the pupil of the eye is at the second focal 
point of the lens, so that the image of the disc in the lens falls on the 
surface of the pupil and more than covers it, the light from the bright 
sectors will appear to be spread out uniformly over the whole field 
seen through the lens. On the other hand, when the lens is nearer the 
disc, the separate white and black sectors will be seen more or less 
distinctly as long as the disc is stationary. If it is in motion, the 
luminosity is just as great, whether the lens is nearer the eye or nearer 
the disc; and hence it is evident that the eye is affected by the inter¬ 
mittent light just as much as it would be by an equal amount of steady 
The truth of the above proposition as far as coloured light is con- 
concerned is shown by Dove’s experiments on the phenomena of 
rotatory polarisation. When double refracting crystals are inserted 
between two Nicol prisms, well-known colour effects are obtained in 
many instances for certain positions of the latter. To some extent 
the colours are distributed uniformly over the entire field, but they 
may also form coloured patterns. In all these phenomena, however, 
the colour of each point of the pattern, as can be demonstrated by^the 
theory of the polarisation of light, will becomé precisely comple¬ 
mentary to what it was at first, if one of the Nicol prisms is turned 
through a right angle. This is experimentally confirmed by turning one 
of the prisms quickly, in which case what the eye sees is white. If a 
coloured glass is also inserted, then for two positions of the prism 90° 
apart the colours are such that together they must give the colour 
of the glass; as is actually found to be the case when the prism is turned 


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