156 



KNOWLEDGE 



[July 2, 1894. 



unearthly cries, which resound morning and evening 



through the forests frequented by these animals, and in 

 imitation of which the hoolock of Assam and Burma takes 

 its name. Although the confined limits of a cage in the 

 Zoological Gardens are far from favourable for displaying 

 the marvellous rapidity of the movements of these 

 creatures — so rapid indeed that birds on the wing are not 

 unfrequently captured — yet the specimen which has been 

 photographed so excellently by Mr. Eanyard indulges in 

 the most frolicsome antics among the ropes and branches 

 placed in its cage, to the manifest delight of all spectators. 

 Equally well marked is the delicacy of touch possessed by 

 the gibbons : this being shown when they amuse them- 

 selves by playing with spiders, which they allow to descend 

 by spinning a thread attached to a finger, and then 

 suddenly jerking them back into their hands, when the 

 unfortunate performers are devoured with much apparent 

 gusto. 



THE DEFINING POWER OF INSECTS' EYES. 



By A. C. Eanyard. 



THE exquisite minute details which the microscope 

 so frequently reveals to us, on examining insects 

 and the minute objects by which they are habitu- 

 ally surrounded, might lead one to suppose that 

 insects can see and appreciate much smaller things 

 than are visible to the eye of man ; but though wa know 

 very little as to what range of sounds insects can hear, or 

 as to their sense of smell, and the other senses they 

 possess, we are able to assert pretty confidently that the 

 defining power of the composite eye of an insect is very 

 inferior to the defining power of the human eye. 



A man with keen eyesight can just distinguish as 

 separate objects adjacent liaes or dots which are separated 

 by an interspace that subtends about a minute of arc. 

 The reader may easily try the expsriment for himself. 

 The lines in the shaded area in Fig. 1 are separated by 

 intervals of about a millimetre from the centre of one line 

 to the centre of the next. Seen from the distance of a 

 foot or fifteen inches, at which he is reading this page, 

 the lines are easily perceived as separate rulings, but from 

 the other side of the room the shading in Fig. 1 appears 

 of a uniform grey tint. At a 

 distance of three thousand four 

 hundred and thirty-eight milli- 

 metres, or about eleven feet, a 

 millimetre subtends an angle of 

 one minute. With a suitable 

 light a keen-sighted person will 

 just distinguish the separate lines 

 at a distance of about eleven feet. 

 I can only distinguish them as 

 separate lines at a distance of 

 about six feet. Hence for a keen- 

 sighted person, in order that two 

 objects may be seen as two, they 

 must subtend at least an angle of about one minute at the 

 eye. For objects which are not suitably illuminated, or for 

 faint stars, a still greater angular distance is necessary in 

 order that we may see them as distinct and separate objects. 

 Thus in « Lyrse, the quadruple star near Wega, the two pairs 

 are separated by a distance of about three minutes twenty- 

 seven seconds, but I only see them as an elongated pair; 

 and I gather from experiments I have tried with children 

 and elder friends that very few people see the two pairs as 

 separate stars, though most long-sighted people see them 

 as an elongated disc, and are able to point out with certainty 

 the direction in which the disc is most elongated. 



Fig. 1. — Lines about 

 millimetre apart. 



There is more than one reason why the limit of distinct 

 vision for the human eye corresponds to about one minute of 

 arc. The retina is a sort of tessellated pavement made up 

 of small organs known as rods and cones, and the spot in 

 the retina which we use when we fix our attention on the 

 examination of an object is termed the forca Intea. In 

 the fared there are only cones packed closely together and 

 separated by intervals of about four millionths of a metre, 

 or about half the diameter of the red corpuscles. The 

 optical centre of the eye lies at a distance of about a centi- 

 metre and a half in front of this part of the i-etina, and at 

 this distance four millionths of a metre, or the distance 

 between the centres of adjacent cones, subtends an angle 

 which is just a little less than one minute; hence, in order 

 that the images of two points of light may fall on the 

 corresponding parts of adjacent cones, their distance apart 

 must subtend an angle of about one minute as seen from 

 the optical centre of the eye. 



There are other reasons, liowever, why the human eye 

 cannot perceive objects as separate objects when they are 

 separated by intervals of less than one minute. As has been 

 pointed out by Mr. -Johnstone Stoney in a very interesting 

 paper published by him in the Scientific Pmceediniis of the 

 B'lijal JJiihlin Society of 20fch December, 1893,'- the inter- 

 ference due to the small diameter of the pupil of the eye 

 causes the image of a point to be represented by a patch 

 or spurious diffraction disc which fixes the minimum risibile 

 corresponding to the diameter of the pupil. 



Thus the angular diameter of the first dark ring seen 

 about the image of a star in a telescope, estimated from the 

 middle of the object-lens, is S = (1'22) ~, where A is the 

 wave-length of the light, and A the aperture or diameter 

 of the objeet-lens. If in this formula we put 6 = 1' = 

 •00029 in circular measure, and A = -6 of a millionth of a 

 metre (which is the wave-length for yellow light), we have 



•00029 = (1-22) |, 

 which gives A = 2521 millionths of a metre, or very 



Fig. 2. — Micro-pUotogriiph of the leiisus of the .compound 

 eye of a Water Beetle. 



nearly one-tenth of an inch, as the diam3ter of the pupil 

 of the eye, which would give rise to a spurious disc of one 

 minute in angular diameter ; and when we scrutinize well- 

 illuminated objects, one-tenth of an inch is about the 



* " Oa the Limits of Vision : with special Reference to tlie Vision 

 of Iniccts," by G. Johnstone Stoney, M.A., D.Sc., F.B.S., Vicc- 

 Preiideut Royal Dublin Society (republished in the Philosophical 

 Jfa</rtjine for" March, 189-i.) 



