THE COMPOUND EYE OF INSECTS AND CRUSTACEA. 
13 
of insect vision, has ripened but slowly during the last thirty 
years : that is to say, since the publication of the researches of 
Johann Muller, in his great work on the Physiology of Man. 
Within the same period, it is true, our knowledge of the 
vertebrate eye has been greatly perfected and extended, as 
indeed was to be expected. Firstly, because our recognition of 
the conditions under which the act of vision is performed by the 
vertebrate organ is founded upon our own personal experience 
of visual phenomena, which necessarily affords a closer insight 
into their nature and causes than can be gained by observing 
the act of vision in the lower animals, whose apparatus of sight 
differs in mechanism and in mode of action, with which we 
cannot familiarise ourselves, as in our own case, by the aid of 
subjective sensation. Secondly, because, in addition to this 
empirical knowledge, the necessity of an intelligent acquaintance 
with the structure of an organ so essential to our well-being 
operates as a constant spur to scientific investigation. Hence, 
also, the study of physiological optics has been pursued with 
the same aim, and in a similar direction : that, namety, which 
chiefly concerns the physical and psychical conditions pertaining 
to the function of vision as exercised in the so-called “ vertebrate 
type ” of eye. 
Long and laborious researches into the structure and function 
of the vertebrate eye have finally been rewarded by the happiest 
results. For, beside that accurate conception of the seeiog 
faculty, and that rational explanation of the visual apparatus, 
which is so important and satisfactory in a scientific point of 
view, we have gathered precious fruits for the service of huma- 
nity, in the many curative methods to which such knowledge 
has led the way. 
On the other hand, the practical interest attaching to the 
study of the eye structures in the lower animals decreases in 
proportion as the apparatus resembles less and less the type of 
visual organ possessed by man. And the labour of investigation 
falls, in this as in every other enquiry of a purely scientific 
character, on those who are content to pursue knowledge for its 
own aims and ends, and to accept every step in advance as suf- 
ficient compensation for their labour. Once attained, however, 
the triumph of this knowledge belongs to and is participated 
by many who follow with lively interest the general progress 
of science. To the readers of our Eeview we may therefore, 
with some confidence, offer a short summary of researches on a 
subject of great interest to all students of natural history. 
The mechanism of the eye as seen in the vertebrate animals 
presents such an obvious relation to its mode of action, that a 
person accustomed to look upon this particular plan of con- 
struction as the only one by which vision can be accomplished 
