Remarks on the examination and classification of cases of colour-blindness / by George A. Berry.

  • Berry, George Andreas, 1853-
Date:
1879
Catalogue details

Licence: Public Domain Mark

Credit: Remarks on the examination and classification of cases of colour-blindness / by George A. Berry. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    way of distinguishing between his cases of red and green blindness. It is based on the principle of simultaneous contrast. When a colourless surface is simultaneously illuminated by a coloured and a white light, the two shadows thrown by an object lying between the sources of light and the surface appear, the one in the colour of the coloured light, the other in its complementary colour. Now, suppose a red and a white light be used for casting the two shadows from the same object, that cast by the white light will be illumined by the red light alone, and will appear red, and that cast by the red and illumined by the white alone will appear green. Red or green blind individuals, not receiving the normal impression from the. one coloured shadow, do not see the other shadow in its true complement, and distinguish the shadows more from their relative darkness than from their difference in hue. For the purpose of being able to vary the strength of these shadows to a measurable extent, Holmgren has constructed his chromatoskiameter, which he thus shortly describes in an abstract of his own paper:1—“ It consists of a stand specially adapted, bearing in the centre an ordinary petroleum lamp; two wooden arms, about a metre in length, proceed from its centre, movable in a horizontal plane round it: these support the pther parts of the apparatus. On the one arm the coloured glass, the shadow-throwing body, and the screen are given a fixed position at a definite distance from the lamp, whilst a mirror reflecting the light of the lamp on the screen is movable back and forward on the other arm, on which the position given to it in any experiment can be read off in a millimetre scale.” Two differently coloured glasses were used for these investiga- tions—a red, which only transmitted red spectral light, and a green, which, besides transmitting all the green light of the spec- trum, also permitted of the passage to a small extent of other rays, but entirely excluded red. The chromatoskiameter has first to be graduated for normal colour-vision—that is to say, the distance found for each glass at which both shadows appear of equal strength, each in its own colour. Holmgren found for himself:— For the red glass, = 40 cm. „ green „ = 35 cm. Difference (R.-Gr.), = 5 cm. He says that it is, as a rule, much easier for the colour-blind to find such a position for the mirror, for this reason, that they gener- ally find the shadows identical in colour when the mirror has attained the position at which they appear of equal strength. This placing of the mirror in the required position is so characteristic that it reveals the nature of the colour-vision. It varies much in 1 Nordiskl Medicinskt Arkiv, vol. xi. No. 6.