556 THE SENSES AND SENSORY ORGANS. 
when Grenacher first published an abstract of his views [216]. 
It would appear at first sight that the evidence brought 
forward by Grenacher must be conclusive, or his views would 
not have been so generally accepted, yet I cannot find a single 
fact in Grenacher’s great monograph [222] which supports his 
theory. Its only basis is the opinion that the great rods 
are composed of elements similar to the retinal rods of the 
simple eyes in Arthropods. Even a cursory glance at Grena- 
cher’s figures is sufficient to render such a statement ex- 
tremely doubtful. The whole theory rests upon this supposed 
identity, and upon nothing else. The great dissimilarity of the 
great rods in different Arthropods, the variable size and disposi- 
tion of the so-called stabchen, the almost protean appearances 
they present under different modes of preparation, more espe- 
cially the diverse characters they present in his own transverse 
sections, are all ignored, and he takes his stand upon a morpho- 
logical presumption. 
Grenacher discusses the manner in which he supposes the 
compound eye has been evolved in support of his theory, 
but attempts no mathematical analysis of the conditions of 
vision. The outcome of the whole theory from a physical 
point of view is as follows. Grenacher says: 
‘If we turn our attention to such an ommateum (Einzelauge) 
we learn which rays are physiologically effective on the rhab- 
dome. It is manifest that a thin beam reaches it, parallel with 
the optic axis. The section of such a beam may be estimated 
by two factors, the curvature of the refractive media and the 
thinness of the connecting thread. When the crystalline cones 
are very pointed, as in Phryganea, the size of this pencil must 
evidently be very minute, as only straight, that is unrefracted, 
pencils can pass through, for which the surfaces of immergence 
and emergence are parallel, which for the apex of the crystalline 
cone must be minimal.’ 
Even admitting that such a narrow pencil hasa physiological 
worth, I would ask, Is it possible that a complex refractive 
apparatus has been elaborated by nature, not to aid vision— 
not even as a useless addition to the eye, but absolutely to 
