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 has a 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 



