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NATURE 



{March 5, 1885 



J. R. Capron, but there seems to be a factor yet unconsidered 

 connected with sharpness of eyesight which is not dependent on 

 the varying aperture of the pupil of the eye. The same amount 

 of light exerts different degrees of stimulus on different indi- 

 viduals, and even in the same person the optic nerves are differ- 

 ently affected, according to his health or age. The pathologist 

 is familiar with the exalted irritability induced by inflammation. 



The observer of close double stars becomes in time painfully 

 aware that through age his power of appreciating minute points 

 of light is blunted, although his eye may be in a healthy condi- 

 tion, and quite equal to microscopic work under suitable illu- 

 mination. 



The flattening of the cornea, together with the slow reduction 

 of the curves of the crystalline lens, is a common occurrence, and 

 this change is said to commence at the age of forty-five. 



Modification of form and the inability to vary the distance 

 between the lens and the retina, due to defective power in the 

 muscles of the iris, are the chief causes of short sight. On the 

 other hand, the eye appears to have great capabilities of modify- 

 ing itself to circumstances. It may degenerate by disuse, and 

 even become obliterated, as may be seen in the blind aquatic 

 beetles of dark caverns, the flea of the bat, and in many species 

 of underground Aphides. Similarly it would seem that the eyes 

 of the student who habitually pores over half-legible German or 

 other type, or the eyes of the watchmaker or the engraver, who 

 use lenses, will permanently accommodate themselves to the 

 short foci required to view objects at short distances, and such 

 modifications may be conceived to become hereditary. 



The pupil of the eye perhaps has an aperture wide enough to 

 admit the pencil of light from any telescope ; yet it may be 

 worth some consideration whether the sensitiveness of the eye 

 may not for certain purposes be increased, under due precau- 

 tions, by the use of some such drug as atropa belladonna. The 

 iris thus might be made less contractile under the overpowering 

 light of a planet, and perhaps allow a better observation of a 

 minute satellite revolving close to its primary. It is a well- 

 recognised fact that a faint star once seen may often afterwards 

 be detected with comparative ease by other persons, if its posi- 

 tion be truly shown. 



Venus may be often seen in broad daylight, if the planet be 

 pointed to by suitable marks. 



Care of course would be taken that the use of belladonna shall 

 not cause the observer to see too much. G. B. Buckton 



The controversy in Nature on this subject has brought back 

 to my thoughts a singular illustration of the power of trained 

 eyesight which seems worth noting, though it does not touch the 

 exact comparison between savage and civilised eyes which is the 

 immediate subject of the letters which have appeared in your 

 columns. I refer to the vastly greater capacity for determining 

 visual direction supplied by the sense of symmetry than by actual 

 discrimination between two slightly distant visible points. If 

 you look at a circle, you can aim at its centre with far greater 

 exactitude than you could aim at a point in the true centre of 

 the figure. Every rifleman and every billiard-player exemplifies 

 this. Suppose a billiard-ball placed a little less than five feet 

 from a pocket, and played at as a half-ball stroke from an equal 

 distance for a winning hazard. This is something like what has 

 to be done from baulk in making a pair-of-breeches stroke into 

 the corner pocket. A fair amateur will pot his ball pretty often ; 

 a first-rate professional will do it very often. No one, perhaps, 

 can make it a really safe stroke. But observe the accuracy 

 required. The margin of error allowed on each side of the per- 

 fect stroke is, on a severe table, not more than an inch at the 

 pocket. This allows an error on each side of about one degree 

 in the point of impact with a radius of one inch (the ball 

 being two inches in diameter). This one inch subtends 

 at the distance from which the stroke is played (nearly 

 5 feet), an angle of i° X sin 6o°, -}$ — about '8'. To make the 

 stroke you must first, by eye, place your striking-ball right, then 

 you must, by eye, aim the stroke right, and finally you must 

 make the muscles follow the eye rightly. These three elements 

 of error combined must leave a resultant error of not more than 

 four-fifths of a minute ; that is to say, a successful stroke must 

 have a total angular error very considerably less than the smallest 

 angular distance which the eye can appreciate between two 

 visible points. This, of course, explains also the superiority of 

 a rifle foresight, which surrounds the object by a symmetrical 

 figure over one which depends on making one point visibly cover 

 another. G. W. H. 



Human Hibernation 



As it is obvious that Mr. A. H. Hulk is unacquainted with 

 the facts of what he designates a "well-known Indian trick," 

 and as the matter is one of considerable physiological interest, I 

 think it well to place before your readers the nature of the 

 evidence which satisfied me of the genuineness of this condition, 

 when I referred to it in the fourth edition of my "Human 

 Physiology," published thirty-two years ago — a reference retained 

 by the present editor of that treatise. This evidence had been 

 obtained by Mr. Braid from Indian sources, and published by 

 him in a collected form in 1850, the greater part of it having 

 previously appeared in the pages of the Lancet. The most 

 important feature of it was the testimony of British medical 

 officers who witnessed the exhumation — most explicitly given in 

 at least three distinct cases — to the corpse-like condition of the 

 liurieci man, a condition which could not be simulated. 



I have since learned from a variety of trustworthy sources, that 

 similar testimony has been over and over again given in India 

 by competent witnesses. Moreover, in one of the ca^es adduced 

 by Mr. Braid, on information supplied to him direct by the 

 British resident in the summer-house of whose garden the man 

 was buried, the circumstances of the inhumation and of the 

 exhumation were such as absolutely to exclude the "tunnel" 

 hypothesis ; while in the case narrated by Lieut. A. Boileau in 

 his "Narrative of a Journey in Rajwarra," 1S35, the man was 

 buried in a grave lined with masonry and covered with large 

 slabs of stone. 



It is further worthy of mention that this performance is not 

 carried on for the sake of gain, but as a religious observance. 

 Many years ago Prof. Max Miiller, finding that I was interested 

 in the matter, kindly placed in my hands a pamphlet printed in 

 India, containing a summary of what is termed the Yoga or 

 Yogi philosophy. The devotees of this system have from time 

 immemorial been in the habit of artificially inducing states of 

 more or less complete abstraction, corresponding closely with 

 those of Braidism ; and the condition of apparent death, in 

 which the soul is supposed to leave the body for a time, for 

 communion with the higher world, is the culmination of these 

 conditions, only to be reached by the few ; to whim, in con- 

 sequence, a character for the highest sanctity attaches itself. 



With the well-authenticated fact of Col. Townsend's self- 

 induction of a state of apparent death, and of his spontaneous 

 recovery from it, as a "leading case," I cannot regard it as 

 incredible that such a condition of "dormant vitality" might be 

 prolonged for days, weeks, or even months, in n warm atmo- 

 sphere. The suspension of the heat -producing power would of 

 course leave the body susceptible of a fatal reduction of tem- 

 perature, if its warmth were abstracted by a surrounding medium 

 much cooler than itself William B. Carpenter 



Athenrcum Club, February 20 



Methods of Determining the Density of the Earth 



I have just seen in the report of the proceedings of the 

 Physical Society (Nature, January 15, p. 260) the account of 

 the ingenious and very important experiments proposed by Drs. 

 Konig and Richarz to determine the density of the earth. I 

 would suggest that mercury be substituted for lead as the attract- 

 ing masses. The homogeneity of density, the precision- with 

 which its density and temperature can be determined, and the 

 ease with which transport from one side of the balance to the 

 other can be effected will commend the use of mercury. The 

 mode of experimenting suggested is the plan of Cornu — used in 

 his determination of the density of the earth by the (Cavendish) 

 Michell experiment — adapted to the same determination by 

 means of the balance. 



Let A, B, C be the balance, D E the attracted balls, and F G, 

 H I, the attracting masses of mercury contained in iron spheres 

 of the same capacity, size, and weight. A large mass of mercury 

 is contained in the vessel M, so placed that it has no effect on 

 the balance or on D or E. The balance being in equilibrium 

 with the mass D and E, mercury is allowed to fill F and I, and 

 the effect noted in oscillations after F and 1 are filled. Then the 

 mercury is drawn from F into H, and G is filled from the reservoir 

 M, and I is emptied, and the second observation obtained. Then 

 D and E are interchanged, and a third observation obtained. 

 Then the mercury in G is run into I, F is filled, and H emptied, 

 and the third observation made, the combination of these four 

 observations making one determination. Electrical effects of 



