Part 5

2. _The Pocket Test._--This consists of nineteen cards, on nine of which are 112 single threads of wool, and 14 pieces of twisted silk, similar to those in the Classification Test. These are numbered consecutively, with the exception of the first thread of the first four cards, and the last thread of the next four cards. The end threads of the first four cards, I to IV, form the tests; they are Orange, Violet, Red, and Blue-green. There are also cards on which red, orange, green, blue, violet and purple, and grey, respectively are to be found. There are also two special cards marked “Without Red” and two special cards marked “Without Green.” (See Fig. 6.)

[Illustration: Fig. 6.]

Many normal-sighted persons might object to the inclusion of some of the colours on the orange card, but this card clearly shows the colours which may be taken as a match. Fine distinctions are not wanted. The series of colours I have selected and arranged so as to confuse the colour-blind and force them to be guided by their colour-perception, whilst the quantity of colour is amply sufficient for the normal-sighted to pick out the colours with the greatest ease. The cards should be arranged irregularly on a white cloth in a good light. The two most important tests colours are the Orange and Violet, Nos. I and II. The person examined should be asked to point out the shades of colour similar to No. I (Orange). A piece of paper rolled to a point should be used for this purpose. If he do this correctly, he probably possesses normal colour-perception. If, however, he match the test with reds or pinks, he is more or less colour-blind, at best belonging to the pentachromic class. If, in addition, he match the Violet test, No. II, with blue, he at least belongs to the tetrachromic class. The trichromic, in addition, may match the Blue-green test, No. III, with brown and grey. The dichromic will match the Orange test, No. I, with yellow-green and yellow-brown. Similar mistakes will be made to those described in connection with the Classification Test. The examinee should be asked to name all the colours on one of the cards. He should also be asked to point out on which of the cards the four test colours are to be found, and which contain none of the test colour.

The examiner should continually change the order of the cards. Most of the varieties of the colour-blind will be readily detected in this way.

The special advantages of this test are: (1) The colour-blind can be ranged definitely in their proper classes. (2) Central scotoma can be detected with its aid. (3) The series of colours are arranged so as to confuse the colour-blind, whilst the normal-sighted easily match the test colours. (4) On account of the introduction of different materials, the relative luminosity and saturation of colours does not serve as a guide to the colour-blind. (5) Portability. (6) The wools and silks are kept clean. (7) An important colour is not likely to be lost.

3. _The Colour-Perception Spectrometer._[13]--(_a_) _Description of apparatus._--This instrument is a spectrometer so arranged as to make it possible to expose to view in the eyepiece the portion of a spectrum between any two desired wave-lengths. In the focal plane of the telescope are two adjustable shutters with vertical edges; the shutters can be moved into the field from right and left respectively, each by its own micrometer screw, and to each screw is attached a drum, the one being on the right and the other on the left of the telescope. On each of these drums is cut a helical slot in which runs an index, and the drum is engraved in such a manner that the reading of the index gives the position in the spectrum of the corresponding shutter in wave-lengths direct. (See Fig. 9.) Thus it will be seen that if, for instance, the reading on the left drum-head is 5320 and that on the right drum-head is 5920, the region of the spectrum from wave-length 5320 to wave-length 5920 is exposed to view in the eyepiece.

[13] Made by A. Hilger, 75a, Camden Road, London, N.W.

[Illustration: Fig. 7.]

[Illustration: Fig. 8.]

[Illustration: Fig. 9.]

(_b_) _Directions for using the instrument._--It should be used as far as possible with a known quality and intensity of light. A small oil-lamp is quite suitable for the purpose. The observer should first ascertain the exact position of the termination of the red end of the spectrum, the left-hand shutter being moved across until every trace of red just disappears. The position of the pointer on the left-hand drum is noted, and the wave-length recorded. The left drum is then moved so that the shutter is more towards the middle of the spectrum. The right-hand drum is then moved, until the pointer indicates the wave-length recorded as the termination of the red end of the spectrum. The observer then moves the left-hand shutter in and out until he obtains the largest portion of red, which appears absolutely monochromatic to him, no notice being taken of variations in brightness, but only in hue. The position of the index on the left-hand drum is recorded. The left-hand shutter is then moved towards the violet end of the spectrum, the right-hand shutter being placed at the position previously occupied by the left-hand shutter. In this way the whole of the spectrum is traversed until the termination of the violet end of the spectrum is finally ascertained with the right-hand shutter. The variation of the size of the patches and the terminations of the spectrum with different intensities of light can be noted. The instrument can also be used for ascertaining the exact position and size of the neutral patch in dichromics, the position of greatest luminosity, and the size and extent of pure colours. When it is used to test colour-blindness, the examinee should first be shown some portion of the interior of the spectrum, and then asked to name the various colours which he sees. In this way he will have no clue to the colours which are being shown him.

_Objections to other tests for colour-blindness._--The tests which have been proposed for colour-blindness are very numerous, but some are so defective that it is rare to detect a single colour-blind person with them. I have, for instance, tested men whom I knew to be colour-blind with certain lanterns with the result that not a single one was detected. In these so-called tests all the requirements of a test and facts of colour-blindness have been neglected. I must, however, refer to three tests constructed by exceptionally able men, each with considerable knowledge of the subject. I refer to the tests of Professor Holmgren, Professor Stilling, and Professor Nagel.

All these tests can be passed at the first attempt without coaching by certain dangerously colour-blind persons, chiefly varieties not known to the inventors, but the chief defect of each is that it is very easy to coach a colour-blind person to pass it. The surgeon to one of our largest railway companies told me that when they used Holmgren’s test they rejected one man in three hundred, but with my lantern twelve in the same number. All these three tests are much better tests when the persons to be examined have not seen them before. A colour-blind man may make only one mistake, say for instance, as in a case I examined the other day, with Nagel’s test (last edition), he passes the test perfectly with the exception of one mistake, that of calling a grey on one card, green. All he has to do is to look for some distinguishing mark on this card in order to go through the test with the ease and certainty of a normal-sighted person. It is the same with Stilling’s letters, he has only to note the letter which he was not able to read and the appearance of the card. A normal-sighted man or woman would readily help him. The confusion of green and grey does not appeal to the average man as a serious defect, especially when he sees his friend go through the rest of the test perfectly. He says to himself, “I suppose he sees a tinge of green in that grey.” The same man would rightly regard it as a most iniquitous proceeding to endeavour to coach his friend through a test when he had seen him mistake a red for a green light.

Holmgren’s test rejects a large number of normal-sighted persons, as may be seen by the reports of the Board of Trade; about 50 per cent of those who appeal are found to be normal-sighted and to have been rejected wrongly.

WILLIAM BRENDON AND SON, LTD. PRINTERS, PLYMOUTH

Reiner and Keeler, L^{td.}

OPTICIANS ----AND---- INSTRUMENT MAKERS

_MANUFACTURERS OF_

The Edridge-Green Colour Perception Lantern The Edridge-Green Classification Test The Edridge-Green Pocket Test and other Optical and Scientific Instruments

THE ABOVE COLOUR TESTS ARE CERTIFIED BY PROF. F. W. EDRIDGE-GREEN

9 Vere Street, Cavendish Square, =London, W.= Telephone: 447 Mayfair.