Studies in the photometry of lights of different colours / by Herbert E. Ives. : spectral luminosity curves obtained by the equality of brightness photometer and the flicker photometer under similar conditions.

  • Ives, Herbert Eugene, 1882-1953.
Date:
[1912]
Catalogue details

Licence: In copyright

Credit: Studies in the photometry of lights of different colours / by Herbert E. Ives. : spectral luminosity curves obtained by the equality of brightness photometer and the flicker photometer under similar conditions. Source: Wellcome Collection.

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    perhaps the influence of the surrounding field, enter in the latter case to cause deviations from the simple conditions. At any rate the simple law which it was hoped to find does not hold for the flicker photometer. The actual effect of this deviation from the simple law is to cause apparent constancy of the flicker curve at moderately high illuminations, since the changes over a long range of intensities are of nearly the order of the error of reading. It must not be forgotten too that these data are all for a large photometric field. Data for a small field are given in fig. 5 by the dotted lines. Their much greater constancy of value is noticeable. A similar test of the method of equality of brightness was made by plotting both the writer's data and Koenig's in terms of log I against log. slit-width, and some iso- chromatics were tried. Here again long curves were obtained. If any simple law holds with regard to these phenomena it is not this logarithmic relation, which, it may be noted, is of course merely Fechner's law. Small Photometric Fields. Fig. 6 shows red and blue critical frequency data for a small photometric field (diameter 10,9). The effect of reducing the field size is to swing the blue line around nearer parallel with the red. which is swung but little. The reversed Purkinje effect will therefore be less, a condition similar to that produced in the flicker photometer by the same change of field size. The most interesting point shown in the figure, however, is the behaviour of the blue at very low illuminations. The violent change of direction which occurs with a large field either is absent or is pushed down to a lower illumination than here tried. The fovea behaves toward blue light just as the larger area of the retina behaves toward red. This is in line with the finding of Koenig that the fovea is blue- blind. It is here found to be not strictly blue-blind but blind to the gray sensation which replaces blue at low illuminations for a larger or peripheral field. There is also some indication that the red small field curve tends to turn at low illuminations in somewhat the manner of the blue curve for large fields. The bearing of these points on colour vision is discussed shortly. An outstanding difference between Haycraft's work and the present research may be noted here. While his results are qualitatively explained by the data of fig. 4, these data would not give his curves. Blue light attains a fixed critical