THE COMPOUND EYE OF INSECTS AND CRUSTACEA. 
17 
cordingly, another mode of vision was propounded to meet the 
seemingly anomalous conditions and deficient mechanism of the 
dioptric apparatus. This new theory was based on the fact, 
since proved to be erroneous, that the minute facets of the 
compound cornea presented perfectly flat surfaces, without and 
within, and that, although the general curve of the cornea 
rendered such eyes capable of a very wide field of vision, no 
collective image of objects was produced by lens action. Each 
separate facet was supposed, therefore, to admit only a central 
pencil of rays, which, penetrating in direct lines, reached the 
ends of nerve fibres from the optic ganglion, and produced 
separate impressions. That is to say, no optical image was per- 
ceived ; but as we see a pattern in mosaic composed of numerous 
inlaid pieces, so the image of an external object was supposed 
to be made up of the separate impressions caused by rays of 
light proceeding from the illumined points of the object seen. 
The concurrence and combination of these separate impressions 
into a picture, formed as it were by the mind’s eye, is there- 
fore a retinal or cerebral function rather than an optical pheno- 
menon brought to pass by physical means. 
To such a theory insurmountable objections present them- 
selves. Anatomical facts, as now interpreted, contradict it ; 
optical phenomena, long known but not sufficiently kept in 
view, disprove it ; physiological reasonings based on the study 
of the true analogies and homologies of the constituent parts of 
the eye compel us to reject it ; and, lastly, direct observation of 
the living organ indicates the closest possible approach to the 
same mode of vision in all eyes possessing a true retina. 
The hypothesis of a double type of construction and function 
of the simple and compound eye was, at the time of its pro- 
mulgation, supposed to be founded on anatomical facts. But 
these were, to say the least, veiy imperfect and too limited for 
so wide-reaching a generalisation. In so far as the word type ” 
may be meant to indicate the existence of important structural 
modifications (not, however, subversive of the law of unity of 
means and purpose), there may be said to be many types of eye 
structure. Strictly speaking, however, they are but variations of 
one fundamental scheme, and cannot be considered as distinc- 
tive characteristics of the respective provinces of vertebrate and 
invertebrate animals : for, in point of fact, the chief variations 
are found in the latter only. In comparing the ascending- 
series, a certain progressive complicity of the retinal structure 
is sufficiently remarkable, but its essential character is the same 
throughout. In respect to the dioptric apparatus, greater 
divergence of plan is apparent in the compound eye ; but this 
is strictly in accordance with the variation of retinal develop- 
ment. But the reason of such modifications is rather to be 
VOL. VIII. — NO. XXX. c 
