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accordance with, the square of the distance is therefore caused 
solely by the divergence of the rays. According to Hotthaft’s 
hypothesis only a single ray reaches each point of the retinula, 
and this consequently retains the same intensity whatever may 
be the distance of the object from which it comes ; and as the 
number of rays which reach the retinula is also constant, its 
whole illumination by the admittedly parallel rays is inde- 
pendent of the distance ; just as the variable distances within 
finite limits of an illuminated body from its infinitely distant 
source of light, have no influence upon the brightness of its 
illumination. (A body that is illuminated by the sun, which 
may be assumed to be at an infinite distance, is, cceteris paribus, 
equally bright, whether I place it on the table or on the 
floor.) 
Secondly, Notthaft’s hypothesis must be pronounced un- 
justifiable, even independently of the above-mentioned error. 
In point of fact an optical apparatus which provides that 
only such light as has strictly the same direction at the optical 
axis shall fall upon the retinula (the surface of which turned 
towards the light has a finite extension) is inconceivable. 
Hotthaft must have overlooked this difficulty and has thus been 
misled to his theory. Moreover, he himself upsets it, apparently 
without being quite clear about the matter, when, after explain- 
ing it, he speaks of the probable purpose of the spherical curva- 
tures of the corneal facets, and on other occasions. Thus he 
himself demonstrates that both converging and diverging rays 
may reach the retinula. But as soon as any ray which does not 
run absolutely parallel to the axis of the ophthalmic element 
can give rise to the sense of light in it, the whole theory falls to 
the ground. The exposition which the author gives of the 
light- separating action of corneal facets is unsatisfactory, for in 
the example cited by him he overlooks the fact that if the cor- 
neal facet acts so as to throw one ray entering parallel to the 
axis to the right and another to the left wall of the crystalline 
cone, a ray falling obliquely to the axis must necessarily exist 
which will attain the apex of the crystalline cone. 
I believe therefore that the theory of ‘ mosaic vision* is 
more firmly established than ever, and that there are no other 
data for the comprehension of the two divergent types of eyes 
than the above-mentioned notion of the superiority of the 
facetted eye in the seeing of movements. Notthaft has main- 
tained that the insect when it moves, for example, when it flies, 
notwithstanding the supposed delicacy of its sensations of move- 
ment, must have difficulty in finding its true position in space. 
I think we may in this case appeal to the birds, which, during 
flight, and consequently while all objects must appear to travel 
past them with greater or less rapidity, nevertheless possess a 
