The detection of colour-blindness from a practical point of view / by F. W. Edridge-Green.

  • Edridge-Green, F. W. (Frederick William), 1863-1953.
Date:
1889
Catalogue details

Licence: Public Domain Mark

Credit: The detection of colour-blindness from a practical point of view / by F. W. Edridge-Green. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    Particular attention should be paid to the answers given to the combination of the thickest neutral glass with the standard red and green respectively. This glass is of such a thickness that it obstructs all light with the exception of a band of red at the extreme left of the spec- trum and a band of green. This glass, when used in combination with the standard red, gives a red light visible to the normal-sighted at a considerable distance. With the colour-bhnd if the red end of the spectrum be much shortened the red light will not be perceived at all. The glass in combination with the standard green gives a dull green light which is easily recognised by the normal-sighted. It will be noticed that with this glass the relative inten- sity and character of the red and green lights are changed. With the unmodified lights the green is lighter and bluer than the red. When modified with this neutral glass the green appears the darker and yellower of the two, exactly as it does in a mist or fog. The two miit colour-blind, therefore, at once call this combination red, because the colour is made to look so much like their red. I have not met with a two unit colour-blind who has named this combination correctly; the answer has invariabty been red usually with some positive exclamation, as There is no doubt about that being red or black. The im- portance of this fact cannot be over-estimated, because I laave tested educated colour-blind persons who have found no difficulty in naming the colours when unmodified with neutral glasses—and so would have obtained a certificate. These would be most dangerous persons at sea, because they would deliberately mistake the red light for the green and vice versa . At the same time they would feel posi- tive about the nature of the lights. It seems to me that in all probability this is how many accidents have oc- curred. If the candidate call the white light, mider any circum- stances, red or green, or vice versa, he is to be rejected. Also if he call the red, green or white hght, black under any circumstances. As it is the confusion of red, green and white which forms gromid for rejection, in examining with the neutral glasses the examiner should be guided by this point. Thus a neutral glass may give a reddish tinge to the light; a