Feb. 26, 1885] 



NA TURE 



or ever from the eyes of the small number of persons whose 

 visual function has been minutely tested, to the properties, as 

 optical instruments, of the eyes of mankind in general. " La 

 position," writes Helmholtz, " des foyers, des points principaux 

 et des points nodaux de l'ceil est assurement soumise a des varia- 

 tions individuelles assez importantes, puisque la plupart des 

 mensurations de l'ceil et de ses diverses surfaces refringentes 

 presentent, chez differents sujets, des differences plus grandes 

 qu'on ne paraissait devoir les attendre pour un organe dont les 

 fonctions semblent reclamer une si grande exactitude de 

 construction." 



As a matter of fact, the theoretical limit of resolving power 

 assigned by Lord Rayleigh, to which he tells us that civilised 

 physicists "approach," is one which cwilised physicist-, have 

 considerably exceeded. The mean of twelve observers, as 

 quoted by Helmholtz, gives resolving power under a visual angle 

 of 101 seconds ; and this mean is reduced by two cases in which 

 the angles were 124 and 147 seconds respectively. The mini- 

 mum was 51 seconds, the most frequent angle was about 80 or 

 90 seconds. The commonly accepted standard of normal vision 

 among civilised people is satisfied by deciphering letters the 

 parts of which subtend visual angles of one minute, while each 

 letter as a whole subtends a visual angle of five minutes. 



I cannot say, however, that I think any such tests are very 

 material to the issue. The eyes of civilised physicists, or of such 

 of them as have undertaken practical research in physiological 

 optics, are probably very highly cultivated, and I doubt whether 

 resolving power, which must greatly depend upon the functional 

 activity of the central depression of the retina or, in the case of 

 stars, u 1 "in the functional activity of the zone which immediately 

 surrounds the yellow spot, furnishes any accurate test of acute- 

 ness of vision in the sense in which I employed the phra c e. 



Assuming the civilised man and the savage to have eyes of 

 precisely equal optical value, the latter might yet possess an 

 acuteness of vision greatly in excess of that of the former ; and 

 this excess might be due to conditions of the percipient elements 

 of the retina which, in the case of the savage, permitted the 

 optical powers to be utilised to the fullest extent. The savage 

 might have greater sensitiveness to variations of light, greater 

 sensitiveness to colour, and acuteness of vision over a larger retinal 

 area. All these advantages might be conferred by better forma- 

 tion or higher development of the retina, and such higher deve- 

 lopment might at once be promoted by exercise and banded 

 down by descent. I support the "commonly-received view" 

 that the vision of savages is more acute than that of civilised 

 men, because this view seems to me to be established by 

 abundant testimony, and to be in perfect harmony with physio- 

 logical knowledge. I feel very strongly that the conditions of 

 town life are unfavourable to the evolution of the eye and 

 favourable to its involution or degradation ; and I believe that 

 a un. .kiate amount of attention might greatly modify these con- 

 ditions, and might do for the eyes what is done by athletic 

 games and exercises for the muscles. 



With regard to the improvement of Lord Rayleigh's own 

 vision, in a dim light only, by concave glasses, I think his 

 ! cannot fail to see that the case, as stated, does not 



contain all the data which would be required in order to arrive 

 at an explanation of the phenomenon. 



R. Brudeneli. Carter 



In a short article on Civilisation and Eyesight which 

 appeared in Nature of February 12, Lord Rayleigh expresses 

 the belief that the greater visual acuity of savages " is a ques- 

 tion of attention and practice in the interpretation of minute 

 indications" and is not ascribable to any possible inherent 

 superiority in their eyes, regarded simply as optical instruments. 

 With this conclusion probably most who have had opportunities 

 of testing the sight of uncivilised races or read the account given 

 who have undertaken such examinations, will agree. 

 The same difference in making more or less out of an imperfect 

 retinal image is met with in different individuals with the same 

 degree of short sight, and otherwise subjected to similar condi- 

 tions according as they have or have not been in the habit of 

 resorting to constant optical coirection of their defect. Such a 

 cerebral elaboration of the retinal image, as it might be called, 

 constitutes also probably the main reason for the difference 

 between the visual acuity of children who have only just learnt 

 to read the letters of the alphabet and adults, which our ordinary 

 tests so frequently show. 



The question of the increasing prevalence of short sight has 

 for a considerable time been the subject of much investigation 

 and speculation in Germany, the results of which have been in 

 many cases to give rise to predictions of rather an alarmist 

 tendency. These, again, have led to legislation in the shape of 

 regulations with respect to school appliances which might meet 

 the theoretical requirements of the most energetic and influential 

 agitators. It is to be hoped that, as the question is now being 

 brought forward in this country, it will be viewed in a more 

 comprehensive manner. The numerous statistics from German 

 schools have slmwn that the proportion of short-sighted boys 

 continually increases from form to form, and from this fact it is 

 very generally argued that the continued use of the eyes for 

 the perception of near objects is the essential if not the only 

 factor in the production of short sight. This view appears, 

 again, to be supported by statistics which allot the largest pro- 

 portion of short-sighted individuals to those branches of industry 

 or those pursuits which constantly call for near vision. Two points, 

 however, appear to be forgotten, or at all events fail to receive 

 sufficient consideration, in arriving at such a conclusion. In the 

 first place, there is an undoubted tendency to increase in the 

 degree of short sight with age alone up to the period of cessation 

 of growth. This has been shown to be due to the elongation of 

 the antero-posterior axis of the eye, which carries the retina 

 further and further from the principal focus of the dioptric 

 media, and is in the vast majority of cases no more a disease 

 than is the attainment of a greater than average height by a 

 certain number of individuals. It is merely a type, and as such 

 is governed by the laws of heredity. A small proportion of cases 

 of sin nt sight are, however, due to disease. These differ from 

 the ordinary cases in that they are seldom hereditary and are not 

 more frequently present in the learned than in the absolutely 

 illiterate classes, besides which the pathological changes to which 

 they are due can often be detected with the ophthalmoscope. The 

 second point which has to be taken into consideration is huw 

 far the greater proportion of short sight amongst literary men, 

 or artisans whose daily work necessitates close vision, is actually 

 due to their occupation, or depends on the circumstance that, 

 being originally short-sighted, they have drifted into pursuits 

 which are more attractive to them, owing to their not being able 

 to enjoy out-door work or sports to the same extent as others 

 whose eyes are more fortunately focussed. That the choice of 

 a life-occupation is often influenced by the condition of the sight 

 is a matter of everyday experience, and it would be interesting 

 to have statistics snowing to what extent this occurs in the case 

 of myopia. Further, as a man's circle of acquaintance is, for 

 the most part, amongst individuals having similar interests in life, 

 intermarriage in myopic families must frequently occur, and 

 would tend to perpetuate, and perhaps increase, the defect. In 

 savages, on the other hand, where the great principle of the 

 survival of the fittest is not frustrated to the same extent as 

 among civilised races, everything would evidently be against 

 the perpetuation of the myopic type. The question comes to 

 be, then, Is not the absence, or comparatively great infrequency 

 of short sight amongst savages due rather to the requirements of 

 such races being antagonistic to the circumstances which would 

 be most likely to perpetuate the myopic type, than to the fact 

 that young savages are not subjected to compulsory education? 

 '1 he pages "f Nature are perhaps hardly the place to develop 

 very fully a epiestion of this kind; suffice it to say, therefore, 

 that the conclusion which such reflections, as well as the result 

 if every-day examination of cases of short sight, appear to 

 justify, is, that the increase of myopia is due mainly to the per- 

 fttuation of a type through the requirements of eh ilisation, and, 

 though not a disease in the ordinary sense, it is desirable to 

 attempt to check its progress. This will assuredly not be an 

 easy matter, but it is not likely to be much influenced by such 

 school reforms as have been introduced into Germany. 



Lord Rayleigh mentions, as an interesting subject for further 

 investigation, the slight myopia which he funis not uncommon 

 when the light is lowered in a room, until objects begin to be 

 indistinctly ^een. He finds, e.g. that though in a good light he 

 sees rather worse with a concave lens of 36 inches focus than 

 without it, yet, when the illumination is diminished, the same 

 lens increases his visual acuity. Altogether, the influence of illu- 

 mination on' visual acuity, and the relation between light-sense and 

 form-sense, are points which have not yet received adequate atten- 

 tion. If the phenomenon described by Lord Rayleigh be really 

 one of short sight occurring under the circumstances mentioned, 

 it is evident that it can only be due to involuntary accommoda- 



