combination all that is required for the durability, comforts and adornment of our public and private buildings.

Thirteenth. I will not increase the list of subjects connected with building further than to suggest improvement in calotype as being peculiarly within the province of the Society of Arts. It is not possible to overrate the growing importance of this subject, an instance of which I have felt lately in my own professional experience, having had some very beautiful specimens sent from Rome, and some also from London, giving the most faithful representations of sculpture that was designed with the view of being used in works on which I am now engaged. This is one instance of how it can be useful; and I have no doubt also, that the time is not distant when there will be no clerk of works in charge of a building of any importance without such a knowledge of the art as will enable him to accompany his weekly report with a photographic representation, illustrative of the progress of the work under his superintendence.

I have thus endeavoured to supply a list of subjects, many of which have hitherto very little, and some not at all, engaged our attention, although coming specially within the objects and provisions of the Society; and I have given them as they occurred to me, having had no time to make any attempt at order or classification. It will, I trust, be our desire during the session to give some of them at least a share of our attention.

I hope you will permit me to conclude by saying, that I have had much profitable experience of the Society of Arts during the time I have been a member, and look forward with confident hope, that during my presidency many valuable communications will continue to be brought before us; and I believe I may also express my confidence, that in the discussion that follows the reading of each paper, whatever the subject of it may be, there will always continue to be maintained that courtesy towards each other, and desire to elicit the truth, which has hitherto peculiarly characterized all your proceedings.

I have only now to ask of you on my own behalf, that in assuming the duties of the Chair in which you have done me the

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honour to place me, the members will extend to me the same support and assistance their kindness has invariably accorded to those gentlemen who have preceded me in the office.

The Secretary moved the thanks of the Society to the President for his excellent address, which were voted unanimously; and on the motion of Richard Hunter, Esq., late H.E.I.C.S., seconded by William Crawfurd, Esq., of Cartsburn, it was resolved to request the President to sanction the printing and circulation of the address amongst the Fellows, to which he kindly consented, though not written with a view to publication.

On Railway and Ship Signals in relation to Colour Blindness.* By GEORGE WILSON, M.D., F.R.S.E., Director of the Industrial Museum of Scotland.

In the account of the researches on colour-blindness which I have had the honour of submitting to the Royal Scottish Society of Arts, I have repeatedly referred to the dangers which might result from the signal-men at railway stations, or on board ship, being colour-blind, and in consequence making mistakes in the exhibition or interpretation of coloured signals. I propose, in the present paper, to enter more fully into this subject, and to consider how the evil may be lessened or remedied. As it is desirable, moreover, that this communication should be complete in itself, I shall commence by dogmatically announcing the chief facts connected with colour-blindness, referring the reader who wishes confirmation of the following statements, or additional information on the subject, to the works mentioned below.†

* Read to the Royal Scottish Society of Arts, 8th January 1855.

1. Extraordinary Facts relating to the Vision of Colours. By Mr John Dalton, Mem. Lit. and Phil. Soc., Manchester. Vol. v. 1798.

2. Wartmann, 1st Memoir on Daltonism or Colour-Blindness, translated in Taylor's Scientific Memoirs for 1846; 2d Memoir in "Deuxième Mémoire sur le Daltonisme. Genève, 1849."

3. Combe's Phrenology, article Colouring.

4. Cyclopædia of Anatomy and Physiology, article Vision.

5. W. White Cooper on Vision. 1853.

6. Dr G. Wilson, Edinburgh Monthly Med. Journal, 1853-1854; or Researches on Colour-Blindness. Sutherland and Knox, Edin.; 1855. 7. J. C. Maxwell, Transactions R.S.E. 1854-55.

I. Of the Nature of Colour-Blindness.

The researches of a great number of the ablest enquirers in England, Germany, France, Switzerland, and America, have, in the course of the last sixty or seventy years, brought to light the existence of a remarkable limitation of vision in reference to colours. This has been variously named, Idiopsy (peculiarity of vision); Chromatopseudopsy (false vision of colour); Dyschromatopsy (bad vision of colour); Achromatopsy (no vision of colour); Dichromic Vision (the vision of only two colours); and Daltonism (vision identical in its peculiarities with that of Dalton); but the terms of Greek origin are not significant to English readers, and are not sufficiently precise and expressive to satisfy even Greek scholars, so that I shall use none of them. Daltonism is an unsuitable and trivial name, so that in referring to the affection of sight under notice I shall restrict myself to the title Colour-Blindness, and call its subjects the Colour-Blind.

Those who are thus named differ from their more fortunate brethren in the following way: Three simple, elementary, or primary colours, properly so called, red, blue, and yellow, are visible by daylight to perfect eyes, besides white, the mutual neutralization of these colours, and black, the absence of them all.*

Perfect natural vision is thus a tricolor, or three-colour vision, and each of the colours of which it is cognizant may be changed by additions of white to it into tints, and by additions of black into shades, without ceasing to be visible till the paleness of the tint has made a very close approximation to white, and the darkness of the shade a very close approximation to black.

Further, the primary colours may be mixed with each other, so as to produce by the addition of red to blue, crimsons, violets, and purples; by the addition of red to yellow, scarlets and oranges; and by the addition of blue to yellow,

*It is assumed here that the analysis of light by our sensations represents its true constitution, and that red, blue, and yellow, are its simplest ultimate sensational elements. This view is the most convenient in discussing the practical relations of colour, but is quite open to criticism as a scientific analysis of light.

greens; all of which secondary colours are visible both when full, and throughout a long series of tints and shades to a perfect eye; as are also the mixtures of the secondary colours with each other, giving russets (including browns) olives, and citrines.

On the other hand, the colour-blind distinguish white and black as perfectly as others do, and a very few of them have no other perception of colour than that implied in the distinction between light and shade.

The great majority however, of the colour-blind distinguish two of the primary colours, yellow and blue, but they err with the third, red, which they confound with green, with brown, with grey, with drab, and occasionally with other colours; and not unfrequently red is invisible to them or appears black. The colour-blind thus possess a bicolor or two-colour vision, so far as the primary colours are concerned.

Moreover, no one of the secondary colours is uniformly visible to them; orange, if tending towards yellow, appears to be yellow; if inclining to scarlet it is mistaken for green; purple is confounded with blue; and green with red and drab and brown. The tertiary colours, such as olive, russet, and citrine, are also confounded with each other, and with green, dark red, and brown.

Further, the lighter tints (among themselves) and darker shades (among themselves) of all colours, primary, secondary, and tertiary, are mistaken for each other, and for white in the case of the tints, and black in the case of the shades, even when not very pale or very dark.

The colour-blind thus perfectly distinguish only two colours, yellow and blue, and these only when deep or full; but as they are liable to mistake purple (or other mixtures of red and blue) for blue, they in reality are clearly cognizant only of yellow.

The identification, confusion, or misinterpretation of colours thus occurring, is most important, practically, in reference to red and green; but colour-blindness is most easily detected by the confusion of certain tints of purple, such as pale violet, lilac, or pink, with blue; and if lilac or pink is thus mistaken by daylight, the other mistakes characteristic of colour-blindness may be regarded as certain to occur.

By ordinary artificial light, such as lamps, candles, and

coal-gas yield, red is less liable to confusion with green than by daylight; and the redder purples cease to appear only blue. Artificial light thus lessens colour-blindness, but does not abolish it, for mistakes continue to be made of the same kind, though not to the same extent as before.

The false, uncertain, defective, or negative vision of colour, which thus characterizes the colour-blind, is compatible with perfect vision in other respects, and is frequently if not generally accompanied by a very nice perception of form and outline not only in full but in faint light.

II. Of the Number of the Colour-blind in the Community. Colour-blindness occurs in all ranks of the community, and in both sexes, but more commonly in males than females. It is congenital, or at least appears as soon as it is possible to test the vision of colours in infancy, and it does not appreciably alter through life, being, so far as is yet known, totally incurable. It is also hereditary, and has been traced without material modification or abatement through five generations. It descends both by the father and mother's side, but always attaches to the sons rather than to the daughters, and if the family be considerable in number, it occurs in more than one of the sons, so that as many as six brothers have been found to be colour-blind.

It varies in degree, the extremer cases being characterized by the confusion of red (in daylight) with black, and the less extreme, only by uncertainty in the lighter and darker tints and shades of the colours characteristically confounded.

The statistics of colour-blindness are as yet imperfect, and do not include females, but there is every reason to believe that the number of males in this country who are subject in some degree to this affection of vision, is not less than 1 in 20, and that the number markedly colour-blind, i.e. given to mistake red for green, brown for green, purple for blue, and occasionally red for black, is not less than one in fifty. The actual number of the markedly Colour-Blind detected in an examination of 1154 males in Edinburgh was one in fifty-five, and the parties thus examined were students, soldiers, and policemen, born in various parts of the British dominions.*

* Edinburgh Monthly Med. Journal, July 1854, pp. 1-101; or Researches on Colour-Blindness, p. 72.