1870.] Dr. Royston-Pigott on an Aplanatic Searcher. 3.27 



muscles attached to the globe which do not admit of separation into 

 distinct parts, but completely surround the posterior half of the globe. 



To trace the optic nerve through its foramen to the brain was success- 

 fully accomplished in only one dissection. After exposing the optic nerve 

 and the eve completely, all the surrounding parts were removed, and a sec- 

 tion made through the skull so as to exhibit a lateral view of the interior of 

 the cranium. 



The brain itself was disorganized in all the young specimens ; but in the 

 dissection just alluded to the optic nerve was seen to pass through the base 

 of the skull, and to enter the membranes to a short distance, so that it 

 would have been possible, if the brain had remained perfect, to trace it to 

 its origin. 



"With regard to the eye itself, no difficulty was experienced in separating 

 the iris, choroid, and lens. The other structures usually existing in the 

 eye had been so long subjected to the influence of the alcohol that I could 

 not determine their condition. 



It must necessarily happen that many interesting observations are made 

 in the course of an investigation like that which has been briefly described, 

 and many minute details might have been added to this account ; but it 

 appeared to me to be desirable to limit the details, as far as possible, to those 

 which were sufficient to establish the remarkable physiological fact that the 

 Mole, at the time of birth, is endowed with organs of vision of considerable 

 perfection, while in mature age it is deprived of the means of sight in con- 

 sequence of certain changes which take place in the base of the skull, ter- 

 minating in the destruction of the most important structures on which the 

 enjoyment of the sense of sight depends. 



II. i! On an Aplanatic Searcher, and its Effects in improving High- 

 Power Definition in the Microscope." By G. W. Roystox- 

 Pigott, M.A., M.D. Cantab., M.R.C.P., F.R.A.S., F.C.P.S., 

 formerly Fellow of St. Peter's College, Cambridge. Communi- 

 cated by Prof. Stokes, Sec. R. S. Received March 31, 1870. 

 (Abstract.) 



The Aplanatic Searcher is intended to improve the penetration, amplify 

 magnifying-power, intensify definition, and raise the objective somewhat 

 further from its dangerous proximity to the delicate covering-glass indis- 

 pensable to the observation of objects under very high powers. 



The inquiry into the practicability of improving the performance of 

 microscopic object-glasses of the very finest known quality was suggested 

 by an accidental resolution in 18G2 of the Podura markings into black 

 beads. This led to a search for the cause of defective definition, if any 

 existed. A variety of first-class objectives, from the T l F to the \ s failed to 

 show the beading, although most carefully constructed by Messrs. Powell 

 and Lealand. 



VOL. XVIII. 2 c 



