86 Mr. A. Mallock. Insect Sight and Hie [Feb. 1, 



only that light coming from the immediate neighbourhood of the axis 

 of the lens to reach the nerve. 



The theory of " mosaic vision " put forward by Johannes Muller 

 has been opposed by some physiologists, who appear to have con- 

 sidered that each lens of a composite eye formed a complete image 

 which was taken cognizance of by the nerves as in the vertebrate eye, 

 and that the whole of these images were in some way added together 

 and arranged by the brain ; I here bring forward some optical reasons 

 which show that Miiller's view is the true one. 



On the supposition, therefore, of " one lens, one impression," the 

 definition obtained by a composite eye will be measured by the total 

 solid angle of view -j- whole number of lenses in the eye. 



The simplest form of composite eye would be a spherical shell, 

 AB, fig. 1, perforated with radial holes, e, c, c, the diameter of these 



FIG. 1. 



holes being small compared with the thickness of the shell. 



If sensitive paper were placed in contact with the inner surface of 

 the shell, it would be impressed with a picture of surrounding objects, 

 for the light which reaches the bottom of any hole is limited to that 

 making an angle less than ^DEF with the axis of the hole, which 

 angle is of course equal to the diameter of the hole -f- half its length. 



It is interesting to see what proportions would have to be given to 

 an eye of this kind if the definition is to be as good as that of the 

 human eye. 



The limit of definition in this case being 1 min., the holes would have 

 to be 7000 diameters long (since 1 min. is nearly 1/3500) and in order 

 that diffraction may not interfere materially with the result,* 



* It may be shown that the hole should not be much smaller than the 

 Huyghens zone of a system for which, if Xjr = r/R, B = the length of the ht 

 X and r being the ware-length of light and the radius of the zone respectively. How 



