434 PHILOSOPHICAL TRANSACTIONS. [ANNO 17 9Q. 



as the present state of our knowledge in this branch of science will admit, no ap- 

 pearance is described which Mr. Ramsden was not satisfied of having distinctly 

 seen. The experiments performed with the single microscope were repeated with a 

 double one, made by Mr. Ramsden, which magnified the object about 40 times; 

 but in the double microscope the appearances were indistinct, the reflexion from 

 the different glasses having thrown a confused glare on the moist surface of the 

 nerve. This circumstance led Mr. Ramsden to object to the use of compound 

 microscopes, and to consider them as unfit for viewing objects of this kind. 



For the following reasons, the optic nerve of the horse was selected, as the 

 most proper for the experiment. It is of a large size, and several inches in length. 

 It is readily procured in a recent state; as there are places in London where horses 

 are allowed to be killed, and regular days in the week are fixed for that purpose. 

 That the examination of the nerve might be made as soon as possible after the 

 animal's death, permission was procured from the man who superintends the 

 killing of horses, to allow Mr. Clift to make the necessary experiments on the 

 spot, the moment the horses were killed. Mr. Clift is the person entrusted with 

 the care of keeping in order the late Mr. Hunter's collection in comparative 

 anatomy, and is well qualified, from his anatomical knowledge, and a familiarity in 

 looking at organized parts through magnifying glasses, for an examination of this 

 kind. These experiments were afterwards repeated by Mr. Ramsden and myself. 

 From this mode of conducting them, the chances of error were few ; since the 

 person who first observed the appearances had no previous opinions on the subject ; 

 and Mr. Ramsden was better able than any other person, to correct such optical 

 errors as might deceive Mr. Clift or myself. 



The first experiments were made on transverse sections of the nerve. One, 

 near its termination in the eye, was placed on glass, and exhibited in the micros- 

 cope the following appearances : it was evidently composed of 1 parts, 1 opaque, 

 the other transparent. The opaque portions were nearly circular in their shape, 

 about 600 in number, and touched each other ; the interstices between them were 

 transparent. When the opaque parts were attentively examined in a favourable 

 light, and the nerve was in a recent state, they were found to be made up of a 

 great number of smaller portions, each of which appeared to be also opaque. To 

 see this subdivision of parts required some attention, and in many sections it could 

 not be perceived. The cause of the difficulty seemed to be, the softness and tena- 

 city of the substance divided, which therefore spread itself over the surface, giving 

 it a uniform appearance : but towards the circumference of the nerve, where the 

 parts were cut obliquely, and some of them tome, the subdivision was very distinct. 

 It was first observed by Mr. Clift, in several different sections; and was afterwards 

 seen very distinctly, both by Mr. Ramsden and myself, in a nerve examined about 

 1 hours after death. 



Having repeated these experiments 6 or 7 times, on different days, so as to 

 ascertain the accuracy of the results, the next object was, to determine whether 



