July 2. 1894.] 



KNOWLEDGE. 



15 i 



diameter to which the pupil of the eye shrinks. Again, 

 the human eye, viewed as an optical instrument, is far 

 from perfect. It is not corrected for chromatic defect or 

 spherical aberration, and such optical imperfections affect 

 the sharpness of the image thrown upon the retina, and if 

 the texture of the retina were finer, and the diameter of 

 the pupil larger, they would alone fix the Hmit of the 

 miiiimuin risihile for human vision at not far from one 

 minute. 



In the composite eyes of insects the retina is convex, 

 with a number of separate small lenses in front of it, 



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Fiu. 3. — Reproduced fro jo a pliotograpli of a uarte-de-visite, taken 

 througli the lenses of the multiple eye of a Water Beetle. 



while in the eyes of vertebrates the retina is concave, with 

 one lens which serves for the whole retina. At last year's 

 meeting of the British Association at Nottingham, the 

 president mentioned in his opening address that the image 

 formed by the compound eye of an insect had been photo- 

 graphed. 



By the kindness of Dr. Spitta I am able to exhibit such 

 a photograph to the readers of Knowledge. Fig. 2 is the 

 micro -photograph of a part of the compound eye of a 

 water beetle {lii/xtuus muniiti'dts), showing the lenses 

 slightly pressed between a microscope slide and a thin 

 cover, so that each of the little lenses is slightly flattened 

 or depressed at its centre, and Fig. 3 shows the multiple 

 image of a photograph as seen through the above mosaic 

 of hexagonal lenses. 



By the kindness of Mr. Gerard Smith, M.K.C.S., 

 I am also enabled to reproduce, for the benefit of our 

 readers, a micro-photograph of the section of the eye 

 of a large fly (Ei-yxtalis tena.v), Fig. 4, which Mr. Smith 

 his very beautifully mounted and photographed. In 

 describing the photograph, Mr. Smith says the little 

 lenses of the cornea can be made out on the surface with 

 a few protective hairs ; each facet forms the base of a tiny 

 cone — the points or apices of all these cones are towards 

 the interior of the eye ; each cone is lined with pigment in 

 such a way that only a microscopic passage is left clear in 

 the centre, through which a ray of light can penetrate 

 and affect the single nerve which enters the apex of each 

 case. Fig. 5 is a more enlarged photograph, also made by 

 Mr. Gerard Smith, of a portion of the same section, 

 and in it the arrangement of nucleated nerve cells can be 



seen forming a stratum around the lower ends of the 

 cones. Arouud each nerve which passes into the apex of 

 a cons there is a group of three large nerve cells, the 

 central one being apparently on the main nerve, and the 

 two outer ones apparently attached by short fibres to the 

 nervous filament along which the optical impression is 

 carried to the brain of the insect. 



Fig. 4 shows in a very diagrammatic form the section 

 of a compound eye, with the lenses, which throw minute 

 images of the objects opposite 

 to them on to the blackened 

 sides of the conical tubes 

 behind them ; a nerve fila- 

 ment leads into the apex of 

 the cone, and carries the 

 optical impression perceived 

 to the insect's brain. 



The simplest form of com- 

 posite eye would be a spherical ^'^'^' *• 

 shell perforated with a number of small radial holes. If a 

 sensitive paper were placed in contact with the inner surface 

 of the shell, it would be impressed with a confused picture 

 of surrounding objects, for the light which reaches the 

 bottom of any hole would be derived from objects situated 

 immediately in front of the hole, and the smaller and 

 deeper the holes the less confused would be the mosaic 

 picture imprinted upon the sensitive paper. Mr. A. Mallock, 

 in an interesting paper recently communicated to the 

 Eoyal Society, and printed in the February number of the 

 Eoyal Society's " Proceedings,' ''• has shown that a spherical 

 composite eye without lenses would need to have a 

 diameter of some sixty-nine feet in order to have a 

 defining power equivalent to that of the human eye, 

 for he shows that the diameter of the holes in the shell 

 should not be less than two thousand wave-lengths of 

 light— say one twenty-fifth of an inch— or difl'raction 

 would materially interfere with the result ; and in order 

 to give a defining power of one minute the thickness of the 

 shell would need to be seven thousand times one twenty- 



FiG. 5. — MitTo-photograpli of a thin vertical seetioji made 

 through the eye of a large Fly. 



fifth of an inch, or twenty-three feet. The radius of the 

 sensitive sphere within the shell is determined by the con- 

 dition that, if the picture is to be continuous, the adjacent 



* " Insect Sight and the Defining Power of Composite Eyes," by 

 A. Mallock. Communicated by Lord Riyleigh, Secretary "of the 

 Royal Society. 



