19 
interior of the globe. The nervous expansion of the retina was the 
only part of the eye cognizant of external impressions, the rest being 
merely a physical apparatus. 
Now, the sensitive retina, only one hundreth of an inch thick in its 
thickest part, consisted of at least seven layers, all consisting of 
nervous matter : the first composed of rods and cones ; then four layers 
of granular matter ; next nerve cells ; and then the optic nerve. 
It was believed that the rods and cones were the percipient part of 
the retina, and for a long time it was held that no similar structure 
could be traced in the invertebrate eye. Miiller, who investigated the 
eyes of both, came to the conclusion that they were constructed on a 
totally different plan, and that there were two types of eye in Nature. 
Within the last few years Leydig and others had come to the conclu- 
sion that there was an unity of type after all. It was a common thing 
in Nature, while preserving the unity of form, to modify the structure, 
sometimes transforming or suppressing unimportant parts, but retain- 
ing all the essential ones, in accordance with the wants and habits of 
the creatures. A dioptic apparatus was not necessary, but the nervous 
mass, with its essential part, the baccillary layer was retained in the 
invertebrate eye. It would be seen that each convex-facet, hemi- 
spherical in the crustacea, abutted on the layer of transparent, pigment- 
covered rods and cones, which were allowed to be the essential parts 
of the vertebrate eye. It would be seen the essential parts were 
retained, viz., the rods, which were a modification of nerve-structure, 
abutting on the corneal facets, just as the baccillary layer in the human 
eye abutted on the optic apparatus, thus tracing out a unity of plan 
though diversity of structure. : 
These remarks had been made to introduce certain slides, one of 
great beauty, lent him by Mr. T. Curties, of Holborn, which cut 
_ through the eye of the death’s head moth, showed the several parts, 
in situ, especially the cones in connection with the corneal facets. 
Seeing the eyes and antennz of insects were their instruments of 
sensation, there was no wonder these apparatus were highly ela- 
borated. 
In proposing a vote of thanks to Dr. HALLIFAX, the President, 
_ Mr. G. Scott remarked that Dr. HALLIFAX had promised them a few 
introductory words, but had, without notes of any kind, given them 
an elaborate lecture. 
Mr. WONFOR said Dr. HALLIFAX modestly attributed the views 
of the connection of the vertebrate and invertebrate eye to others, 
