44 PROCEEDINGS OF SOCIETIES. 



the only part of the oyc cognizant of external impressions, the rest 

 of the eye being merely a physical apparatus. 



The sensitive retina, only one hundredth 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. 



Eecently, Leydig and others had come to the conclusion that there 

 was a unity of type after all. It was a common thing in nature, 

 while preserving the unity of plan, to modify the structure, sometimes 

 transforming or suppressing unimportant parts, but retaining all the 

 essential ones, in accordance with the wants and habits of the 

 creatures. Thus in the invertebrate eye the dioptric apparatus was 

 not necessary, but the nervous mass, with its essential part, the bac- 

 cillary layer, was retained. It would be seen that each convex facet, 

 hemispherical 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. Thus it was seen the essential 

 parts were retained, viz. the baccillary layer, which was a modification 

 of nerve structure, abutting on the corneal facets, as the rods and 

 cones in the human eye abutted on the optic apparatus, thus tracing 

 out a unity of plan with diversity of structure. 



He had made these observations for the purpose of introducing to 

 their notice certain slides, more especially one 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 con- 

 nection with the corneal facets. Seeing the eyes and antenna? of 

 insects were then instruments of sensation, it was not wonderful these 

 apparatus were highly elaborated. 



In proposing a vote of thanks, the President remarked that Dr. 

 Ilallifaxhad 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, whereas, 

 whatever others had done, to his knowledge he had at least six years 

 ago worked out the views he had enunciated to them, and, moreover, 

 pioneered the way to making sections of eyes prior to that time. 

 Some years since he explained his method of making sections of 

 insects and their parts to the " Quekett Microscopical Club," and 

 a section of insect's eye on the stage of his own microscope, showing 

 the parts in situ, was made by Dr. Hallifax at least five years ago. 



The meeting then became a conversazione, when the slide above 

 mentioned, together with sections of the eyes of prawn, shrimp, 

 crayfish, crab, lobster, moths, flies, &c, made by Dr. Hallifax, were 

 shown by him, and by Messrs. Hennah, E. Glaisyer, and Wonfor. 



