158 



KNOWLEDGE 



July 2, 1894.] 



holes must just be in contact at the internal surface of 



the shell ; that is to say, the diameter of the hole must 

 subtend one minute at the centre of the spherical shell, 

 or the sensitive surface must have a radius of eleven feet 

 sis inches. 



If, still keeping one minute as the limit of definition, 

 we substitute the arrangement actually found in composite 

 eyes, and in place of the long tunnels in a thick shell we 

 use short tunnels or cones with a lens at the outer end of 

 each, and a diaphragm at the inner end, pierced with a 

 small central hole, the proportions of the eye will be 

 determined in the first place by the diameter of the lens 

 which will just define one minute, and secondly, by making 

 that diameter subtend one minute at the centre of the 

 sphere. 



The size of the image of a point formed by a lens (as 

 seen from the optic centre of the lens) is inversely as the 

 diameter of the lens, and it takes a lens four inches in 

 diameter to define one second of arc, /.c, to separate points 



KiG. ft. — Mii-i-o-photograpli made with m liigli jiohci- of \ci-lical 

 section through the eve of n Fl v. 



one second apart ; hence the lens which will just define one 

 minute is /„, or 0-066 of an inch in diarneter, and the 

 radius at which 0066 of an inch subtends one minute is 

 about nineteen feet. Mr. Mallock therefore concludes that 

 no composite eye of practicable dimensions, acting as 

 supposed above, could be made to give definition even 

 approiching that of the human eye. 



The great advantage of the composite eye to insects lies 

 in the fact that the closer they are to an object the better 

 will be their sight of it, for the greater will be the number 

 of lenses employed to produce the impression ; whereas in 

 the simple eye, the greater focal length of the lens 

 mconveniently limits the distance at which a distinct view 

 can be obtained. Insects evidently see all distant objects 

 in a very difi'used and badly defined manner. The 

 impression perceived must be something like a picture 

 executed in very coarse wool-work — or very rough mosaic — 

 near and distant objects being equally m focus ; and it is 

 only when an object is brought close to the eye, so that it 

 covers an area corresponding to several facets of the eye, 

 that its outline would be defined. Mr. Mallock has 

 measured the diameters of the facets and of the composite 

 eyes of many insects, and gives the following interesting 

 table in'licating the probable dpfin'no; power of their 



vision. It -will be seen that the blow-fly and the dragon- 

 fly are keen-sighted as compared with the bees and 

 butterflies. 



Notias of Boolts. 



'J'lie Faunn of tlw Dirp Sea. By Sydney J. Hickson, 

 M.A. Cantab, et Oson., D.Sc. London. (Kegan Paul, 

 Trench, Triibner it Co., 1894.) Dr. Hickson remarks that 

 the bottom of the deep sea was, until recently, quite an 

 unknown region. It was regarded by most persons, when 

 it entered into their minds to consider it at all, as one of 

 those regions about which we do not know anything, never 

 shall know anything, and do not want to know anything ; 

 but during the last quarter of a century the English, 

 French, German, ItaUan, Norwegian and American 

 Governments have vied with one another in despatching 

 expeditions, well equipped for exploring the dark regions 

 beneath the sea, and a vast store of information is to be 

 found in the scientific reports of these expeditions which 

 have been issued. Dr. Hickson's book brings together, in 

 a small compass and in a very interesting form, many of 

 the more important conclusions which have been arrived 

 at, as well as the facts on which these conclusions are 

 based, and the whole is presented in a form which will be 

 readily understood by those who do not possess a specialist's 

 knowledge of genera and species. 



The deep sea seems to be absolutely dark, so far as 

 sunlight is concerned. The temperature is only a few 

 degrees above the freezing point, and the pressure is 

 enormous. There is little or no movement of the water. 

 The bottom is composed of a uniform soft mud, and there 

 is no plant life. 



