Improving the blackboard / by W. Douglas Seymour.

  • Seymour, W. Douglas.
Date:
1938
Catalogue details

Licence: In copyright

Credit: Improving the blackboard / by W. Douglas Seymour. Source: Wellcome Collection.

    19/42 page 15
    same illumination. This was done by placing first a white screen 2 feet square at a distance of 3 feet (i.e. to subtend a visual angle similar to that subtended by a child’s paper or a blackboard) from the subject’s eye, which was then photographed. The experiment was then repeated with a black screen. Despite the fact that the illumination of each screen was the same, the difference in pupil size, as measured on the photographs (Fig. I) with the aid of dividers, amounted to nearly 20 per cent. While this part of the experiment was being carried out, it was found that similar work was being done (for a different purpose) at the National Physical Laboratory with the far greater resources of its Photometric Laboratory. The results1 obtained there were kindly put at the dis¬ posal of the present writer. B. H. Crawford, who was responsible for this work, measured the variations in pupil size of ten subjects when subjected to fields of diff¬ erent brightnesses and subtending an angle of approximately 60 degrees. Large variations of brightness were used and wide individual differences were found. Within the range of brightness difference produced by a blackboard and white paper with the same illumination, the difference of pupil size was quite marked ; in the first case the mean pupil diameter of ten subjects was 4.48 mm., in the second the corresponding figure was 3.87—a difference of .61 mm. or 16 per cent. The pupil sizes quoted are those likely to result from the difference between the reflexion factors of blackboard and white paper under the ordinary conditions of class¬ room lighting. It may be noted that other workers (Lythgoe,2 Reeves,3 and Holladay,4) found even greater differences. These changes in pupil size are unlikely in themselves to con¬ stitute a source of strain. The time taken for such a change is quite considerable but, as can be easily demonstrated, visual discrimination is possible before the pupillary adaptation is complete. It is rather the concurrent retinal adaptation which may be considered the more serious problem, and a reduction in this would doubtless have beneficial results in the saving of time and in the reduction of fatigue. The prob¬ lem of this change of visual state must be approached by considering the 1 Subsequently published as “The Dependence of Pupil Size upon External Light Stimulus under Static and Variable Conditions”. Proc. Roy. Soc., B. No. 823, Vol. 121. 2 K. J. Lythgoe. Report Med. Res. Council. X. “Measurement of Visual Acuity.” 3 P. J. Reeves. Opt. Soc. Amer. Vol. 4. 4 L. J. Holladay. Opt. Soc. Amer. Vol. 12.
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