THE MODE OF ACTION OF FACETTED EYES. 
339 
Johannes Muller’s theory of ‘mosaic vision/ and, adopting 
Gottsche’s interpretation, regarded each facet of the compound 
eye as an independent, perfect eye, possessing its retina and 
retinal picture. 
In 1871, Boll,* chiefly supported by anatomical facts, first 
suggested a return to the old Mullerian views. He had re- 
marked that, under favourable circumstances, small pictures of 
external objects can also he obtained through the rods (bacilli) 
of the retina of the newt, and his attention was then called to 
the physiological insignificance of such pictures. In point of 
fact, every fat- globule under the microscope shows compara- 
tively sharp pictures. 
Grenacher, who had published a preliminary statement of 
the results of his investigations in the Gottinger Nachrichten ,/• 
also opposed the interpretation of the facetted eyes in Gottsche’s 
way, which had become general, and expressed himself in 
favour of Muller’s theory. A year after the appearance of this 
preliminary statement, and unfortunately without any know- 
ledge of it, I subjected the prevalent theory to a critical exami- 
nation of a more physical character,]; by studying the optical 
properties of a special example, the eye of the Hydrophilus piceus. 
It appeared to be exceedingly questionable whether the picture, 
which was supposed to be a retinal picture, was at all to be re- 
garded as such optically ; and further, that this picture, and 
especially that of the fly which Gottsche studied, does not exist 
at all during life. It is produced only when the crystalline 
cone is removed, and this was done by Gottsche. During life, 
when the crystalline cone lies behind the corneal facet, it cannot 
be produced. 
On the contrary, I showed that the corneal facet and 
crystalline cone provide in the most remarkable manner for the 
bringing together, at the bottom of the crystalline cone, of a 
comparatively large number of the rays issuing from that point 
of the object that lies in the axis of a facet. The corneal facet 
acts in this case as a lens ; and the crystalline cone by total 
reflection, as a catoptical apparatus. The rays which penetrate 
into it nearly parallel to its axis are, as it were, caught and 
conducted by reflection to its apex. Other rays, which enter 
at greater angles, may in part, by repeated reflections be re- 
jected again without reaching the apex of the cone ; part of 
them will pass the walls of the cone and be absorbed by the 
surrounding pigment, seeing that, at each reflection, the angle 
under which they strike the surface of the cone becomes greater. 
* Du JBois-Reumond und Reichert's Archiv. 1871. 
t 1874. 
% TJeber das Sehen von Beivegungen, und die Theone des zusammen- 
gesetztes Anges, Wien. Akad. Sitzungsb. 1875. 
