( lviii ) 
selves actually by millions on the electric lights established in 
some of the small towns of America. Forel’s experiments on 
varnishing the eyes of insects also seem to point to the 
probability that perception of the direction of light is a main 
factor in guiding movement. Diptera, whose compound-eyes 
he covered with an impenetrable varnish, did not, when first 
released, direct their movements in any definite direction, 
but ended by flying straight up in the air quite out of sight, 
—that is, in the direction in which they would still perceive 
light by their ocelli or simple eyes. 
If the direction of some particular light, or amount of light, 
play the part of a mariner’s compass in guiding each flight 
of an insect, and if it possess an accurate perception of lights 
and shades by means of its facets,—that is, perceive the 
direction in which some shade cast on its eye is most intense, 
and that in which itis least intense,—it could avoid an object 
perfectly, although never perceiving anything more than a 
part of the shadow affecting the optic organ, and by which the 
source of light was partially, or more or less completely, 
eclipsed. Toa creature of this kind refractions of light will 
be most important, and one of the most pressing series of 
considerations about the compound-eye will be its properties 
as a set of refracting instruments. 
The multiplicity of facets must clearly be a great advantage 
in the perception of movement. Between the insect’s eye 
and some set of rays of light a shade commences to appear ; 
at the first moment one set of facets is affected, at the next 
instant adjoining facets are occupied; surely not only may a 
movement make itself thus felt by the ocular organ, but also the 
direction of the movement may be determined. I have already 
said that Exner advocates the seeing of movement as one of the 
functions of the insect’s eye. Patten has criticised in an un- 
favourable manner Exner’s views as to the perception of motion, 
but Icannot myself see the force of his arguments, if it be 
understood that itis not the lights and shades of a retinal picture 
that are affected; the whole cmmatidia* are, as I put it, in 
equilibrium with the amount of light falling on them, and each 
may have a separate perception of a change in this amount. 
* The ommateum of recent anatomists. 
