330 Dr. G. J. Stoney on the Limits of Vision. 



parts of the field of view. Accordingly, in one or other of 

 these ways, or by a combination of them both, the insect may 

 be able to adjust one part of its field of view for near objects, 

 and other parts for more distant ones ; e. g. y a fly may be able 

 to view distant objects around with the utmost distinctness of 

 which its eye is capable, at the same time that it is closely 

 scrutinizing the details of a lump of sugar and applying its 

 proboscis rapidly to one minute crystal after another. As 

 the adjustment which would enable it to do this would be 

 of service to the insect, and as the construction of its eye 

 admits of it, it seems likely that it is one for which pro- 

 vision has been actually made. 



On a review of the whole subject we seem to have a satis- 

 factory general insight into the process by wdrich vision 

 through compound eyes is carried on. Doubtless much de- 

 tailed information of the minute anatomy of these interesting 

 structures has been reached by microscopic anatomists ; but 

 I am not acquainted with it, and have been obliged to rely 

 on my own imperfect observations. It is, however, likely 

 that, notwithstanding the diligence of microscopists, much 

 still remains to be explored ; and this, I hope, may be fol- 

 lowed up more intelligently if the general optical process is 

 understood. It is on this account that I have endeavoured 

 to trace it out, and especially because among my scientific 

 friends there are to be found some of the most competent 

 persons thoroughly to explore the whole of this interesting 

 subject. 



I have hitherto said nothing about vision through the 

 isolated eyes with which insects are also furnished. They 

 cannot, from the minuteness of their lenses, give them nearly 

 so good vision of distant objects as man enjoys. And the 

 limit is very possibly still more restricted by their being 

 furnished with but a moderate number of rods and cones. 

 It would be of interest to ascertain by observation whether 

 this is so, and to collect such data as w r ould enable us to 

 estimate with tolerable exactness how far the imperfection 

 goes. 



[Note, added February 21. — It is not obvious why the apparatus of 

 rods and cones in the human eye is nearly on as small a scale, and as 

 closely packed over the rest of the retina as in the fovea Ititea, since the 

 amount of detail we can see in the corresponding parts of the field of 

 view is immensely less. Possibly the line of the embryonic development 

 of this layer may be such that it could not be evolved of the requisite 

 minuteness in one part, without being made nearly as minute over the 

 whole. The only other possible explanation seems to be that its being 

 minute serves some other unknown purpose, as well as removing one of 

 the three obstacles to our vision of small details.! 



