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XXV. 
ON THE LIMITS OF VISION: WITH SPECIAL REFERENCE 
TO THE VISION OF INSECTS. By G. JOHNSTONE 
STONEY, M.A., D.Sc., F.R.S., Vice-President, Royal Dublin 
Society. 
[Read DecemBer 20, 1893; Received for Publication Frsruary 1; Published 
FEBRUARY 23, 1894.] 
CONTENTS. 
PAGE 
InTRoDUCTORY REMARKS, : 5 : ‘ 6 : 228 
Srcrion I.—Of Vision in general, . ; ‘ : 228 
Srcrion II.—Of Vision with compound Eyes, ‘ : : 237 
Inrropuctory REMARKS. 
Tue President of the British Association, at the recent meeting of 
that body in Nottingham, mentioned in his opening address that 
the image formed by the compound eye of an insect had been — 
photographed. This suggests the inquiry how the image is 
formed, and what is the limit of the vision of which it is the 
physical basis. ‘The investigation of this point shows that insects 
cannot see very minute objects, and the whole inquiry seemed of 
sufficient interest to be laid before the Royal Dublin Society, 
especially as it suggests much further study which the author 
could not attempt, but which there are other members of the 
Society most competent to undertake. 
Section I.—Of Vision in general. 
As preliminary to the inquiry it is well to consider what are 
the causes that limit the amount of detail that can be seen by the 
instrumentality of eyes such as our own, 
the kind of eyes of which we know most. 
That there is such a limit to human vision 
may be easily seen by placing a well- 
illuminated ruling of parallel lines at 
different distances from the eye of a per- 
son whose visionis good. Let us suppose 
black lines ruled, as in figure 1, on a 
Fic. t—Mirumetric Ruuine. white surface at intervals of one milli- 
metre from the middle of one line to the middle of the next. If 
