134 On the Optical Advantages Bn 
beautiful plexus, a large portion of which is, however, destined for 
the innervation of the iris. From this course plexus bundles of 
nerve fibres dip into the muscle, in which they form a finer net, 
from which single fibres of extreme tenuity are traceable for long 
distances amongst the muscular bundles, but I have not yet disco- 
vered the actual nature of their ultimate connection with the mus- 
cular fibre. In my last course of lectures I adverted to the occur- 
rence of ganglion cells in this plexus. They first became known to 
me by the beautiful preparations of Schweigger, and are not the 
coarser gangliform swellings recognizable under slight enlargement, 
described by Dr. R. Lee, jun. The arteries of the ciliary muscle 
are drawn from the circulus arteriosus major iridis, which dis- 
tributes many recurrent twigs to it. There are not unfrequently 
offsets of the arterioles, which this arterial circle sends to the ciliary 
processes. ‘The venous blood escapes in two directions, posteriorly 
through veinlets, which join those of the ciliary processes, and lead 
to the ven vorticoss, and in front through veinlets which empty 
their contents into the circulus venosus in Schlemm’s Canal. 
IV.—On the Optical Advantages of Immersion Lenses, and the Use 
of Deviation Tables for Optical Research. By Royston-Pieorr, 
M.A., M.D. Cantab., M.R.C.P., F.C.PS., F.R.AS., formerly 
Fellow of St. Peter’s College, Cambridge. 
Part IT. 
Puate LX, 
In the first part of this paper I had the honour of drawing the 
attention of the readers of this Journal to the relative quantity of 
rays composing a nascent pencil emanating from an illuminated 
particle immersed in Canada balsam, in the two cases either when 
a film of water or when air intervenes between the covering glass 
and the facet lens of the objective ; and I ventured to offer a compa- 
rative Table of Deviations, which rays of light entering the front 
surface of the objective had already suffered according to the media 
of transmission: up to 50° the deviation appeared three times less 
vid water than vid air. The angle of total internal reflexion in the 
two cases being— 
For plate-glass and air, 41° 48’ 
», water, 62° 57’ 
” ” 
When objects are mounted in the dry way instead of Canada 
balsam, every deviation is changed both for dry and immersion 
