453 



NA TURE 



[March 19, 1885 



of many rapidly succeeding impressions, then tints are graded 

 into one another at the edges, and we lose the power of dis- 

 tinguishing detail. 



I can give, fortunately, a case in point. My eyes are affected 

 with a small amount of astigmatism. It does not affect general 

 vision for ordinary purposes, nor, of course, the definition of single 

 lines ; but, when I use appropriate lenses, the whole scene 

 becomes brighter and more cheerful, and I see details. The 

 bark of a tree is a perfectly different object with and without 

 them. With them it is like a go >d photograph ; without them, 

 like many pictures. Formerly, in addition to the cylindrical 

 surface, I required a slight spherical concave, and I was disposed 

 to place the increased general brilliancy of the view mainly to 

 the reduction of size, but I now use piano-cylindrical lenses for 

 distant vision, and it is evident that the brilliancy is solely due 

 to the better definition. 



I would, lastly, suggest for Lord Rayleigh's consideration the 

 question whether the change of focus of his eyes in faint light 

 is not partly, at all events, due to change in the colour of the 

 light. I know that there is such a change with me, but 1 

 have long had reason to believe that colour affects my vision. 



J. F. Tennant 



37, Hamilton Road, Ealing, W., February 7 



Those who have compared Lord Rayleigh's letter in Nature 

 of February 12 with that of Mr. Brudenell Carter on February 

 26 will have observed an inconsistency occasioned by a slip of 

 the pen. 



The latter says : ' ' The commonly accepted standard of normal 

 vision is satisfied by deciphering letters the parts of which sub- 

 tend visual angles of one minute. ..." Also, Prof. McKendrick 

 states that " The smallest visual angle in which two distinct 

 points may be observed is 60 seconds." 



According to Lord Rayleigh, however, " A double star cannot 

 be fairly resolved unless its components subtend an angle exceed- 

 ing that subtended by the wave-length of light at a distance equal 

 to the aperture. If we 'ake the aperture of the eye as i/sth inch, 

 and tlie wave-length of light as l/40,000th inch, this angle 1, 

 found to be about two minutes." In the case of a small angle 

 the aperture divided by the distance is approximately equal to 

 the arc divided by the radius or to the circular measure of the 



, tt , , i/'-ro.oooth inch 



angle. Hence in the present ease we have ^ 



I/Sth inch 



I ,■ 206,26? 



— radian or — ~ — 2 



8000 Sooo 



25 'S seconds nearly, instead of the 



two minutes accidentally stated by Lord Rayleigh. 



This minimum value seems to show some mistake in Ehren- 



berg's experiments on vision, and is about half of that found by 



Helmholtz for the best of twelve observers. 



March 10 Sydney Lupton 



[Mr. Lupton is quite right. By a stupid blunder I said about 



two minutes, when I should have said about half a minute. — 



Rayleigh.] 



There is a defect of eyesight common among the natives of 

 India known as " ratandhi, " lit. "night blindness." Persons 

 affected with this have either ordinary powers of vision by day- 

 light, or else powers su lit lie less than ordinary as to feel no 

 inconvenience, so that usually no defect is noticeable ; whilst in 

 feeble twilight their sight fails in the most extraordinary way, 

 and in the dusk they become (in bad cases) practically blind. 

 Of course there are all degrees of this affection ; but the strongly- 

 marked cases alone are likely to attract attention. 



By medical men in India this affection is said to occur most 

 among men living on a low diet (chiefly of cereals), and the 

 usual palliative treatment is to prescribe a meat diet. 



This affection is rarely noticeable among Europeans in India, 

 though I have sometimes noticed marked differences of clearness 

 of sight among them also amounting to slight "night-blindness. 

 Lord Rayleigh's case of short-sightedness in twilight and in the 

 dusk seems to be a mild case of this sort (see Nature, February 

 12, p. 340). Allan Cunningham 



The Pupil of the Eyes during Emotion 

 Although further obervations are required, there seems to 

 be a more or less general assent as to the influence of the emo- 

 tions on the pupils of the eyes. Mr. Clark, in his letter to your 

 journal (vol. xxxi. p. 433), has rightly quoted Gratiolet, who 



says that in sudden astonishment or fear the whole system be- 

 comes paralysed, and at the same time the pupils dilated. In 

 anger, on the other hand, when the whole body is roused into 

 action, the pupils become contracted: " Les pupilles sont 

 enormement dilate : es dans l'epouvante, tandis qu'elles sont 

 toujours contractees dans le colere." This was, however, said 

 many years before by the celebrated Harvey, who, in his dis- 

 course on the circulation of the blood, written in 1628, says : 

 " In anger the eyes are fiery, and the pupils contracted" (" Ira 

 rubent oculi, constringitur pupilla"). 



I should myself think that a narrow pupil evinces a more 

 active mental state, as it is this condition which is present when 

 the eye is accommodated to regard with attention a near object, 

 whilst, on the other hand, when gazing out into distance, the 

 pupils are wider, and the mental mood is more passive and 

 contemplative. 



In my parrot the size of the pupil is a very excellent measure 

 of its frame of mind. When angry the pupil becomes minutely 

 contracted, whereas when the bird is sympathetic and amiable 

 the pupils become as widely dilated. Balzac, with other 

 novelists, have depicted the state of the pupils when describing 

 the various emotions and passions. The former in pourtraying 

 a saintly woman kneeling before the altar, says : " The pupil of 

 the eye, endued with great contractility, appeared then to 

 expand and draw back the blue of the iris until it formed no 

 mure than a narrow circle. What force was that arising in the 

 depths of the soul which so enlarged the pupils in full daylight 

 and obscured the azure of those celestial eyes ? " Darwin 

 speaks doubtfully, but rightly demands more observations on the 

 subject. Samuel Wilks 



Grosvenor Street, March 



Aurorae 



After a long and remarkable absence of aurora, which, from 

 a letter in your columns of February 19 (p. 360) does not appear 

 to have been confined to these more southerly latitudes, we were 

 favoured last evening with a beautiful, though somewhat tran- 

 sient display. It was about 9/25 p.m. when I first noticed a 

 long band or belt of light above the northern horizon. At first 

 it «as ill-defined, with little change of position, but in about 

 twenty minutes it became more luminous and the characteristic 

 streamers suddenly made their appearance, shooting upward-, 

 sometimes from above, sometimes from below the belt of light, 

 which for a few seconds changed into a double arch. Some of 

 these streamers rose as distinct columns, showing the usual 

 prismatic hues, one in particular being noticeable as traversing 

 the inverted W of Cassiopeia, another forming a fan-like 

 terminus to the luminous region, but all confined to a low alti- 

 tude, bounded on the north-west by Perseus, and on the north- 

 east by Vega, then rising. It may be well to observe that on 

 the same day (the 15th) a large sun-spot had just reached the 

 central meridian, and was beginning to show signs of great 

 disturbance. E. Brown 



Further Barton, Cirencester, March 16 



Injuries caused by Lightning in Venezuela 

 In answer to Mr. von Danckelman's inquiry as to the use ot 

 lightning-rods and the frequency of accidents from lightning in 

 the tropics (Nature, December 11, 1SS4, p. 127), I beg leave 

 to offer the following information referring to Venezuela, where 

 I have been residing ever since 1862 : — 



Thunderstorms are very frequent during the rainy season. 

 They break out generally in the afternoon, about the time of the 

 daily maximum of heat, whilst they are extremely rare in the 

 morning (I only witnessed one case) and during the night. 

 Statistics of accidents do not exist, nor are there many lightning- 

 rods in use (in Caracas about half a dozen). But there are 

 certain regions where the former are far from being uncommon, 

 as, for instance, the country around the Lake of Valencia and the 

 plains or Hanoi to the north of the Orinoco. In these a consider- 

 able number of cattle are killed by lightning every year, and I know- 

 also of several cases where houses were destroyed and people 

 killed. The herds of cattle crowd together as soon as a 

 thunderstorm begins, and the animals remain during the whole 

 time with their heads down to the ground, thus avoiding in- 

 stinctively that their pointed horns should act as lightning- 

 conductors. 



In the neighbourhood of Maracay, at the eastern end of the 

 Lake of Valencia, accidents occur almost every year. A very 



