Colour-Blindness

With a Comparison of Different Methods of Testing Colour-Blindness

Mary Collins

• Publisher: Psychology Press
• Year: 2015
• eBook ISBN: 9781317358213
• Edition: 1
• Topic: Psychology
• Subtopic: Applied Psychology
• Index: Psychology

Originally published in 1925, this book embodies the results of research on red-green colour-blind subjects, supplemented by brief accounts of blue-yellow, total, and acquired colour-blindness to complete the description of the different forms of the defect. After a historical survey of previous work by such men as Dalton, Helmholtz, Rayleigh, Edridge-Green and others, the author deals with the most important theories of colour-blindness, and with a description of the tests and a discussion of their results.

Chapter I
Introductory

THE study of colour-blindness has been somewhat retarded by the concomitant study of colour theories. The majority of investigators have started out unduly biased by their favourite theory and have examined colour-blinds from this prejudiced standpoint. The result is that a great deal of unnecessary confusion has gathered round this subject, which has ultimately caused an obscuring of the real issues. Attempts have been made from time to time to get away from theories, but, on the whole, these have proved futile, and the results achieved are only gradually permeating the literature of the subject. Yet even in the highest authoritative references to colour-blindness, it is amazing to find that a description of the defect is inevitably given along the lines of some particular theory, although it must be admitted that some authors take the precaution of inserting a note to the effect that such a description holds only if that theory be accepted.

Dr Hayes, who revolts against this method, states that " the general topic of colour-blindness is still in a state which many psychologists consider to be most disgraceful to their science. One reason for this backward condition is undoubtedly to be found in the extreme complexity of the subject, and the enormous variations from case to case; but an even greater obstacle to the progress of knowledge has been the almost universal practice of studying and classifying cases under the domination of some preconceived colour-theory."¹

The division of partial colour-blinds into Red-blind, Green-blind, and Violet-blind, following the Young-Helmholtz theory, is the most noted example of this falsifying of results. It is a more or less established fact that the so-called red-blind is also blind to green, and the so-called green-blind, blind to red, yet this terminology is still frequently applied to describe this colour anomaly. And what is more surprising is that in detailing tests useful in examining colour-blinds the tests are vitiated by the theory lying behind them. The Holmgren wool test, a splendid one for dichromates, is still seemingly employed to select out the red-blinds from the green-blinds, and Holmgren's coloured plate showing the matches which the two types of colour-blinds make is often referred to as authoritative. We still find, in so recent a book as Abney's Researches in Colour Vision, instructions similar to Holmgren's own. " If in the second test, he selects with purple only green and grey, or one of them, he is completely green-blind. The red-blind never selects the colours taken by the green-blind, and vice versa."'² This division of the colour-blinds into groups, which are totally at variance with the known facts, has done much to spread false ideas of the defect, and progress has been considerably impeded. The wool test, as we shall see later, is excellent for diagnosis if we leave aside the implications on which it was based and reject the idea of a rigid classification.

The Helmholtz theory has been merely cited as an illustration of a theory predominating facts. There is no intention to decry the theory as an explanation of the various phenomena of colour defect. It has been modified since Young and Helmholtz first formulated it. But what must be emphasized is that its influence is clearly visible in the reports of many of the cases of dichromasy which have been investigated.

Brief Historical Survey

It is interesting to trace the illumination which has been gradually thrown upon this curious defect. As the experiments which are to be described later deal only with colour-blindness in its most common form, namely, blindness to red and green, only a brief account will be given of blue-yellow blindness and of total colour-blindness.

1. Red-Green Colour-Blindness

The earliest case on record seems to be that of Harris, the shoemaker, reported in 1777 by Mr Huddart in a letter to the Rev. J. Priestley. Mr Huddart writes " The account he (Harris) gave was this: that he had reason to believe other persons saw something in objects which he could not see; that their language seemed to mark qualities with confidence and precision which he could only guess at with hesitation, and frequently with error. His first suspicion of this arose when he was about four years old. Having by accident found in the street a child's stocking, he carried it to a neighbouring house to inquire for the owner. He observed the people called it a red stocking, though he did not understand why they gave it that denomination, as he himself thought it completely described by being called a stocking." This seemed to imply a blindness to red, although Abney explains the defect as being one of green-blindness. " He observed also, that when young, other children could discern cherries on a tree by some pretended difference of colour, though he could only distinguish them from the leaves by their difference of size and shape. He observed, also, that by means of this difference of colour, they could see the cherries at a greater distance than he could, though he could see other objects at as great a distance as they; that is, where the sight was not assisted by the colour." This seems to be the first scientific account of any abnormality in colour vision, although it must always have been a fairly common defect; evidently it had existed undetected. This first account appears to be a description of a case of confusion of both red and green. It is interesting to note that Harris had two brothers who were similarly affected.

In 1794 Dalton's. description of his own case appeared¹ and attracted considerable attention—so much so that Daltonism became for long the name by which colour-blindness was described. He first became acquainted with his defect by observing a pink geranium in candle-light. " The flower was pink, but it appeared to me almost an exact sky-blue by day. In candle-light, however, it was astonishingly changed, not having then any blue in it, but being what I called red—a colour which forms a striking contrast to blue." He found on examination that his brother suffered from the same defect—showing, as in the case of Harris, the hereditary nature of the phenomenon. He further states that while he found that most people could distinguish six colours in the solar spectrum, his colour sensations were reduced to two, blue and yellow, or at the most three—blue, yellow, and purple. " My yellow comprehends the red, orange, yellow, and green of others; and my blue and purple coincide with theirs. That part of the image which others call red appears to me little more than a shade, or defect of light; after that, the orange, yellow, and green seem one colour, which descends pretty uniformly from an intense to a rare yellow, making what I should call different shades of yellow. The difference between the green part and the blue part is very striking to my eye; they seem to be strongly contrasted. That between the blue and purple is much less so. The purple appears to be blue, much darkened and condensed." This is an excellent description of a colour-blind. Dalton's defect was said to be due to an inability to see the colour red, and Daltonism accordingly sometimes stands for this particular form of colour-blindness. Dalton attributed his defect to the fact that one of the humours of his eye, probably the vitreous humour, was a colour medium, probably some modification of blue. But an examination of his eye after death did not support any such theory.

Goethe in 1812 in his Theory of Colours¹ described this defect of colour-blindness in the following manner. "We will here advert to a very remarkable state in which the vision of many persons is found to be. As it presents a deviation from the ordinary mode of seeing colours, it may be classed under morbid impressions; but as it is consistent with itself, as it often occurs, may extend to several members of a family, and probably does not admit of cure, we may consider it as bordering only on the nosological cases." The two cases which Goethe was acquainted with saw white, grey, and black in the usual manner. They also saw yellow, red-yellow, and yellow-red, but they confused blue with pink, green with dark orange, green and brown, " These persons saw fewer colours than other people, hence the confusion of different colours."

The next cases reported are in 1816-7 by Dr Nichols— one of a boy aged eleven, the other of a man aged forty-nine. They are both reported as making the typical mistakes of the red-green colour-blind. " The colour I am most at a loss with is green; and in attempting to distinguish it from red, it is nearly guess-work. The different shades of red and green, I know not to which they belong; but, when they are before me, I see a difference in the shade. Though I see different shades in looking at a rainbow, I should say it was a mixture of yellow and blue—yellow in the centre and blue towards the edge."

In 1837 Seebeck gave a detailed analysis of the several cases which he had investigated. He found he could divide his subjects into two classes based on the difference in the length of the spectrum which was visible to them. This led later to the classification of deuteranopes whose colour system is reduced to blue and yellow, but who have a normal length of spectrum, and protanopes who likewise see blue and yellow but whose spectrum is shortened at the red end. This is the most marked differentiation of the present day and has been generally accepted.

Sir John Herschel, however, was the first to put forward the dichromic explanation of colour-blindness. He pointed out in his well-known article on " Light," written for the Encyclopedia Metropolitana,¹ that certain individuals could only distinguish two colours, blue and yellow. This seems to have been the first positive statement of the diagnosis of the defect—the former cases reported being merely descriptive. Herschel considered Dalton's case as coming under this category, but this explanation was objected to by Prof. George Wilson. Wilson had taken particular interest in Dalton's case, and considered that Dalton did not show signs of blindness to red.

In Wilson's book, Researches on Colour-Blindness (published in 1855), he describes a large number of cases which he had personally studied. He discusses the various theories which had been formulated at that time and examines many of the phenomena which had been observed to accompany the defect. He points out, for example, that some Daltonians, as he calls them, can distinguish colours by other means than vision, " Slight differences in shape, accidental rough points, folds, wrinkles, and the like " and touch. " Wools dyed with certain compounds are much harsher to the touch than those dyed with others; the mineral pigments, such as Prussian blue, or chromate of lead, in general producing rougher surfaces than the organic dyes, such as indigo. A wool dyed with a mineral might thus be distinguished by the touch, from one dyed with a vegetable red, although the colour-blind eye could detect no difference between their tints."¹ An important step was taken by Wilson in testing the colour-blinds. He used a large number of samples of coloured wool, coloured paper and glasses, which the examinee arranged in groups—this is the method so splendidly adapted later by Holmgren in his investigations.

Clerk Maxwell in the same year, in a communication to the Royal Society of Edinburgh, on " Experiments on Colour as Perceived by the Eye, with Remarks on Colour-Blindness," seemed to agree with Prof. Wilson in regarding dichromic cases of colour-blindness as not firmly established. " In experiments made with the pure spectrum, it appears that, though the red appears much more obscure than the other colours, it is not invisible." He adopted a new method of experiment, by revolving colour discs and forming colour equatio...

Table of contents

• Cover
• Title
• Copyright
• Original Title
• Original Copyright
• Contents
• PREFACE
• INTRODUCTION
• CHAPTER I — INTRODUCTORY — BRIEF HISTORICAL SURVEY
• CHAPTER II — COLOUR THEORIES WITH SPECIAL REFERENCE TO COLOUR-BLINDNESS
• CHAPTER III—DESCRIPTION OF TESTS AND DISCUSSION OF RESULTS
• CHAPTER IV—TESTS AND RESULTS (continued)
• CHAPTER V—TESTS AND RESULTS (concluded)
• CHAPTER VI—A BRIEF DIAGNOSIS OF THE INDIVIDUAL CASES
• CHAPTER VII—CONCLUDING REMARKS
• BIBLIOGRAPHY
• INDEX