596 
MR B. T. LOWNE ON THE MODIFICATIONS OF 
by each facet. It will be seen by the accompanying figures (figs. 45 to 55) " that the 
curves of the meridians of the compound cornea approach more or less closely to an 
epicycloid. 
The average angles subtended by the facets in the region of most distinct vision in 
different insects are given in the foliowing table, in which I have added the greatest 
angles subtended by the facets at the periphery, the diameter of the corneal facets and 
the acuity of vision, both in the centre and at the periphery ; according to Snellen’s 
system, the unit of vision being the power of perceiving an object at twenty feet, 
which has a diameter corresponding to an angle of five minutes : an angle of one 
minute being taken as the mean size of a visual perception in man. The signification 
of the fraction in the fourth and fifth columns is, that an object appears in the same 
detail to the insect as it does to man, when the distance of the object from the eye is 
* See description of plates, p. 602, 
