BY THE PRESIDENT. 
113 
of vision is still an unknown problem. Johann Muller,* in 1826, 
propounded the so-called mosaic theory. He supposed that each 
facet produced a single visual impression of obj ects in their natural 
position, and that the whole visual field consisted of a mosaic of 
such impressions. R. Wagner, j* in 1835, suggested that the com¬ 
pound eye was an aggregate of simple eyes, and that the whole 
visual field was a mosaic of reversed and inverted images. Wagner, 
Ruete,J and Dor§ mistook for the retina the capsule of the crystal¬ 
line cone; and various writers, viz. Max Schultze,|| Claparede,U 
Grenadier,** Hickson, ff and quite recently Patten,JJ ascribe to it 
a position between the cornea and the inner end of the great rods. 
It is, however, generally held that each facet takes in a small 
portion of the field, which is inverted and reversed as in a camera- 
obscura, and that the insect combines the separate images to form 
one picture. But how ? Let us suppose that the rays of an 
external object which we will take to be an arrow fall upon three 
facets of a compound eye. The image of the arrow would be 
inverted and reversed as below. 
A <-;--- 
B -—> - >»-- 
A. External object. B. Three images of the same inverted. 
It is not easy to understand how an insect by means of its nervous 
system could combine the inverted images so as to represent the 
arrow. The difficulty of correction would be so great that Muller 
was led to believe that the visual impressions originated by each 
facet were neither reversed nor inverted, but that the mosaic pattern 
on the retina must consist of objects in their natural position. 
I have already stated that the generally-accepted opinion is 
that the long rods are terminations of the optic nerve. Gottsche§§ 
suggested that they were highly refractive axial threads. Lowne, j| || 
however, holds that the spindle which they contain and which corre¬ 
sponds to that found in the “compound single eyes” of noctuid 
* ‘ Zur vergleichende Physiol, des Gesichtsinnes des Menschen und der Thiere,’ 
Leipzig, 1826, and a translation by Baly, ‘ Physiology of the Senses.’ 
t Wiegmann’s ‘ Archiv,’ Bd. i, p. 372. 
X G-ratulationsschrift der med. Fac. zu Leipzig zu C. G. Caras, 1881. 
§ ‘ Archiv d. Sciences Phys. et Natur.,’ 1861. 
|| Schultze’s ‘Archiv,’ Bd. ii, p. 404. 
IT ‘ Zeitschrift fair Wissenschaftliche Zoologie,’ Bd. x. 
** ‘ Untersuchungen iiber das Sehorgan der Arthropoden,’ Gottingen, 1879. 
ft “ Eye and Optic Tract of Insects,” ‘ Quarterly Journal of Microscopical 
Science,’ vol. xxv (1885), p. 215. 
XX “ President’s Address, ’’ ‘ Transactions Entomological Society,’ 1888, p.xlviii. 
§§ Muller’s 6 Archiv,’ 1852, p. 483. 
|j|| Zoo. cit. 
VOL. V.—PART IV. 
8 
