438 



NATURE 



{Sept. 5, 1889 



distance very similarly to the case related by Mr. Griffith. One 

 curious feature in the present instance was that the roots of the 

 free could be traced to a considerable distance by the earth above 

 them being thrown up as over a mole barrow. 



Most of the trees struck here have been oak ; but there were 

 also two poplars, four elms, a chestnut, and the fir above 

 mentioned. It is said that beech trees are never struck : 

 probably ihe smooth close-fitting bark makes a better conductor 

 than the rough bark of the oak. J. P. Maclear. 



Cranleigh, August 26. 



No5p-9I^ckening as a Preventive of Snow-Blindness. 



• In vol. xxxviii. of Nature there were several interesting 

 letters on this subject. Will you allow me to suggest a possible 

 explanation ? • 



For some years past I have interested myself in the choroidal 

 circulation, and my observations have led me to believe that 

 when light is absorbed bj the choroidal pigment the blood- 

 supply at that spot is increased. If the light is intense, this 

 increase soon has the effect of blurring the image, and if at the 

 same time the light is intense and the exposure to it prolonged, 

 the sensitiveness of the retina may suffer for some time after from 

 the same cause, i.e. an abnormally large blood-supply in the 

 choroid. 



In the course of the blood-vessels there is just such a con- 

 nection between the retina and the skin as the nose-blackening 

 preventive requires. My suggestion is that the blackening of 

 the skin increases its demand for blood in some way, perhaps 

 by its increase of temperature, and that thus a larger supply is 

 drawn along the main branches of the ophthalmic artery, the 

 naso-frontalis, the supra-orbitalis, and the lacrymalis, diminish- 

 ing the quantity which finds its way into the small and almost 

 independent system of the choroid. In this way nose-blacken- 

 ing would save the retina from being oppressed and injured in 

 the way mentioned. 



I may mention that if anyone after walking for an hour or so 

 in the snow covers his eyes so as to exclude all external light, 

 he will find his eyes filled with a very bright retinal light, and 

 also if he is at ail accustomed to see the blood corpuscles moving 

 in his field of vision he will see them at such a time very 

 di>tinctly and in great numbers by looking at the sky. 



The theory which my observations have led me to form, that 

 there is a veryintimate connection between the. retinal light and 

 the circulation' in the choroid is almost necessarily crippled by 

 the fact that it rests largely upon subjective phenomena which 

 are misleading, are not demonstrable, and depending on the 

 constitution of the suliject are not readily confirmed. Until, 

 therefore, I can hear of another worker in the same field whose 

 observations agree in the main with my own, I do not feel 

 prepared to put^lish them. Henry Bernard. 



Jena, August. 



A Method of Mounting Dried Plants. 



• An example of a very useful and expeditious method of 

 mounting herbarium specimens adopted here is inclosed as 

 worthy of attention. Short strips of lead, used in packing tea, 

 are passed through slits in the paper on each side of the part of 

 the plant to be fastened, and the ends then bent flat out on the 

 back of the sheet. The many advantages of using this, or some 

 other pliable metal, in certain cases, are very obvious. Has this 

 method been hitherto suggested ? John Wilson. 



University, St. Andrews. 



COLOUR-BLINDNESS AND DEFECTIVE FAR- 

 SIGHT AMONG THE SEAMEN OF THE 

 MERCANTILE MARINE. 



IN the House of Commons, recently, attention was 

 called to this subject by Dr. Farquharson, who, in 

 stating that he would take an early opportunity of dis- 

 cussing it next session, intimated that the efficiency of 

 the Board of Trade regulations on this matter was open 

 to grave suspicion. On making inquiry, we find his 

 doubts are only too well founded. When, in the year 



1852, the carrying of red and green side-lights by sailing- 

 vessels was made compulsory, the subject of colour- 

 blindness had not awakened the attention of practical 

 observers. Had the fact that from 3 to 4 per cent, of 

 the whole male population are colour-blind then been 

 known, it is possible that some mode other than by show- 

 ing red and green lights would have been devised to 

 indicate the positions of vessels at sea at night. As there 

 is generally but a hazy conception of what is meant by 

 the term colour-blind, we will briefly indicate its exact 

 significance. 



When, in 1794, the distinguished chemist, Dalton, pub- 

 lished a description of his sense of colour, the scientific 

 world were surprised to find that (here existed individuals 

 whose perception of colour differed in a remarkable way 

 from that of their fellow-man. To have said that an indi- 

 vidual possessed the sense of sight was tantamount to say- 

 ing that he possessed the sense of colour, the latter being 

 considered an integral part of the former ; but Dalton's 

 report clearly showed that the two senses were separate 

 and distinct, and that, while an individual might have a 

 perfect appreciation of form, he might also be quite un- 

 able to perceive any distinction between two or three 

 or more distinct and different colours. Further investiga- 

 tion showed that there were a few people who could dis- 

 cern no colour at all, every object appearing as black or 

 white, or as shades of black and white (grey). This is 

 total colour-blindness, and is very rare. The usual form, 

 and that which we allude to when we speak of a colour- 

 blind, is that in which the individual can distinguish the 

 colours blue and yellow, but can see no difference between 

 the colours red, green, and brown; andfrom the fact that one 

 of these individuals, if given a vivid scarlet skein of wool, 

 will select to match with it green skeins and brown skeins, 

 it follows that he must see green and scarlet as he sees 

 brown. Now, there being between 3 and 4 per cent.^ of 

 the whole male population afflicted with this variety, it 

 follows that a very large section of the community are by 

 nature disqualified for all those positions in which the 

 correct interpretation of coloured lights is essential to 

 safety. Clear as this fact must be, it was not until Dr. 

 George Wilson, of Edinburgh, in the year 1855, published 

 his admirable work, entitled " Researches on Colour-blind- 

 ness," that public attention was invited to the subject. 

 He showed with the greatest clearness how the safety of 

 a vessel lay in the hands of men — "look-outs," officers, 

 and pilots — who might be colour-blind, but were un- 

 conscious of this defect, or afraid to confess it ; and he 

 came to the definite conclusion, as the colour-blind 

 were in a minority in the community, therefore, those 

 destined to deal with signals should be selected solely 

 from the majority whose vision was normal, and he 

 earnestly urged upon those in authority the necessity of 

 excluding colour-blind men from the sea profession. 



One sentence was prophetic, that in which he says 

 "the appalling yearly list of lost vessels which appears 

 in our Wreck Returns awakens the suspicion that more 

 than one of these fatal disasters may have resulted from 

 the mistaken colour of a lighthouse beacon or harbour 

 lamp, which on a strange coast, and with the accompanij 

 ments of a snow-storm or a thick fog, has been wronglj 

 deciphered by a colour-blind pilot." ^ And if true of th 

 pre-steamship days when vessels carolled along at tl: 

 rate of a few miles an hour, what is to be said of the pre 

 sent time, when our " greyhounds " of the ocean flas' 

 along at the rate of twenty miles an hour, day and night 

 The regulation red and green lights of a steamer ar 



' Holmgren examin-d 32,165 men: 1019 co'oir blmd— 3168 percent. Ja 

 Jeffries examined 10,387 men: 431 olour-blind— 4'i49 per cent. LondO 

 C.immittee examined 14,846 men : 617 c )lour-bIind— 4"i56 per cent. 



2 Colour-blindness proved to be the cause of Lvinbemian and Isaac Set 

 collision : ten lives last. Colour-blindness proved to be the cause of loss of 

 City c/ Austin— colour of buoys was mistaken : money loss ^40,000. Colour- 

 blindness or defective sight was the cause of collision between Carbet C-' " 

 and/. //. Rainier: money loss p^igoo. 



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