Journal,

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 discovered the actual nature of their ultimate connection with the muscular fibre. In my last course of lectures I adverted to the occurrence 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 distributes 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 venæ vorticosa, 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-PIGOTT, M.A., M.D. Cantab., M.R.C.P., F.C.P.S., F.R.A.S., formerly Fellow of St. Peter's College, Cambridge.

Part II.

PLATE 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 comparative 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 viâ water than via air. The angle of total internal reflexion in the two cases being

When objects are mounted in the dry way instead of Canada balsam, every deviation is changed both for dry and immersion

lenses. The nascent pencil successively encounters variable resistances, and finally, in the water lens, attains much less obliquity of transmission.

And, as explained in the first part, an immersion objective of 80° aperture gathers as large a nascent pencil from a shining par

ticle Q as a dry objective of 120° aperture: the great bulk of the nascent rays passing more centrically through the water film.*

I will avail myself here of some known results of the immersion system.

Dr. Woodward could only effect the resolution and enumeration of Nobert's XIXth Band by means of the Powell and Lealand Immersionth, not by the dry lens. These opticians cannot show the markings of the Acus or Amphipleura pellucida except by the immersion lens, which, as already stated, is able to gather up a volume of oblique rays nascent from the illuminated particle via water, which are impossible via air (for a Canada-balsam mounted object); the quantity of nascent rays, and which are really the best defining rays, being, as compared with air refraction, as 24 to 1, to be shown farther on.

Again, if the minute spherular bodies detached from diatoms. especially those scattered in the vicinity of Angulata-be steadily examined both with the dry and immersion lens of the finest quality, the confused halo disguising the true definition of the particle as a luminous fog is much less apparent in the water lens. The

* The course of each of the rays is laid down by the Deviation Tables.
VOL. IV.

L

beading is blacker. A broken line of beads is given with unusual sharpness as black as jet. The light playing in the focus of the minute refracting bead or lens, conforms more closely to the phenomena of a brilliant point-it swells and contracts with a change of focus-and its colours are transformed. We cannot yet see the black ring which undoubtedly surrounds the bead in nature, when viewed singly, but crescentric black shadows are formed en masse; the halos of each spherule becoming commingled together, and the jet black shadows then start forth through the general haze into an exquisite tracery of shaded silicious beading.

The results obtained in this paper would seem to strongly justify Hartnack's observations. He asks, "Are large apertures an advantage to the microscopist or to the optician?" and declares "the disadvantages are for the latter only." It is clear that if by the aid of a water or other fluid film, a larger nascent pencil of rays can be transmitted to the eye by means of a smaller apertured objective, whose corrections are far less expensive and difficult than for the glass of enormous aperture, then the advantage of destroying the excessive deviations caused by the aëro-refraction is selfevident.*

To find the angle of total internal reflexion of a nascent ray originating from a brilliantly illuminated particle under different conditions of optical refraction, from a denser medium into a rarer, as from glass into air, water, or oil of turpentine, it is only necessary to take the largest angle, which causes the emergent ray to verge upon an angle of 90°, whose sine is unity. If, therefore, this angle be called the limiting angle of refraction, since

Giving therefore μ the values ascertained for refractions between the given substances, ' the limiting angle of refraction at which total internal reflexion takes place is readily ascertained, as shown in the Appendix by Logarithmic Tables.

* Nobert's lines are cut upon the under surface of the covering glass, and therefore the defining rays proceeding from a point in the groove may be considered as immersed, or rather formed in the glass so as to produce the same effect nearly as a body mounted in balsam. On this account water lenses, or lenses immersed in other fluid (especially such as have a refractive index nearly equal to that of the covering glass), transmit extremely oblique nascent rays throughout more centrically than the dry.