In order to ascertain the probable error of the mean principal incidence and azimuth as determined with Plate 1, the measurements were repeated with Plate 6; the difference between the two values of the principal incidence, according as one or other of the neutral axes of the plate was in the plane of incidence, being least, and therefore the retardation for red light differing least from 90° for Plates 6 and 7.

Similar measurements, which were about as concordant, were made with the gold plate in water and carbon bisulphide. The numbers in the table being the means of eight observations, four with the principal section of the polarizing nicol inclined to the right, and four with it to the left of the plane of incidence.

The mean values of the principal incidence and principal azimuth obtained with the two quarter undulation plates being different, it was assumed that the errors of the means are as the squares of the small errors of the plates, and that the errors of the incidences in either position of the plate, and therefore the algebraical differences or numerical sums of the errors in the two positions, that is, the differences of the apparent principal incidence in the two positions, as the first powers; and therefore that the errors of the means are as the squares of the difference of incidences in the two positions.

Gold in Air.

Principal incidence...

azimuth....

Difference of principal inci

о

dence in two positions... 9 28 or 568, 4 18 or 258.

-22

Thus the residual corrections to the results got with Plate 6 will be to the difference on the results got by Plate 1 and Plate 6, as 2582 to 5682-2582, or as No. log 1·41491 to 1; this gives +6′ and 5′ making the corrected principal incidence and principal azimuth 76° and 35° 27'.

In a similar manner the means of the results got with the gold plate in water and carbon bisulphide were corrected, the final results being with red light.

Principal incidence.

Principal azimuth.

In order to determine the principal incidence and azimuth for gold by an independent method, the one originally used by Sir David Brewster was adopted; the quarter undulation plate was removed, and a second gold plate attached to the vertical stage in such a manner that, whilst the plates remained parallel to each other, the distance between them could be altered. The plates were so adjusted that

when the light was incident upon the surface of the first at angle of about 70°, it was reflected once by either plate.

The incident light being polarized in a plane inclined at an angle of 45° to the plane of incidence, the position of the stage and of the analysing nicol were altered till the reflected light was reduced to a minimum.

A rectangular glass trough was placed on the horizontal stage of the goniometer so as to surround the gold plates; the trough filled with water, and the principal incidence and the azimuth observed.

The ray of light which had been twice reflected by the plates being parallel to the incident ray, and the trough having been placed with its front perpendicular to the direction of the incident light, the polarization of the ray could not be altered in any way by the glass, as indeed was verified by experiment.

The light having been twice reflected, the square root of the tangent of the angle which the plane of polarization of the reflected ray makes with the plane of incidence, is equal to the tangent of the principal azimuth.

The principal incidence and principal azimuth determined by this method from eight observations, four with the plane of polarization of the incident light on either side of the plane of incidence are—

The principal incidences agree fairly well with those obtained by the other method; but the azimuths are somewhat higher.

The following table contains the values of these constants for gold in air, as previously determined by—

Sir David Brewster.

("Optics," ed. 1853, p. 309, 311)..

70 45

33 0 for jewellers' gold.

Professor Haughton.

(“Phil. Trans.,” 1863, p. 81). .... 75 37 G. Quincke.

("Pogg. Jubelband," p. 336)

.....

72 47

Assuming that the tangent of the angle of principal incidence is the index of refraction of the metal for red light, the value of that angle in air, as deduced from the measurements made in water and carbon bisulphide with the quarter undulation plates, is 76-53 and 77.22 instead of 76°.

The numbers given by Quincke ("Pogg. Ann.," vol. cxxviii, p. 541) for silver are-

In air...

In water

In turpentine

Principal incidence.

Principal azimuth.

The value for the principal incidence in air calculated according to the same assumption, by multiplying the tangent of the principal incidences in water and turpentine by the refractive indices of these substances, is 75° 55′ and 75° 36′ instead of 74° 19′; in all four cases the value is too high.

Although more experiments are required to decide this point, it seems probable that this relationship between these numbers is not merely an accidental one; and if so that there is additional reason for adhering to Sir David Brewster's opinion that the value of the angle of principal incidence may be taken as indicating the refractive power of a metal.

In conclusion, I must express my thanks to Professor Stokes for much advice and assistance, and specially for pointing out the method for determining the residual corrections to the results obtained with the quarter undulation plates.

January 16, 1879.

W. SPOTTISWOODE, M.A., D.C.L., President, in the Chair.

The Presents received were laid on the table, and thanks ordered for them.

The following Papers were read:

I. "On some Points connected with the Anatomy of the Skin." By GEORGE THIN, M.D. Communicated by Professor HUXLEY, Sec. R.S. Received November 25, 1878.

[PLATES 2 and 3.]

Rollett, in 1858,* in a memoir on connective tissue, described the results of an elaborate investigation into the structure of the corium. Microscopic examination of leather, and of skin tanned by himself, had shown him that the connective tissue bundles of the corium are made up of smaller divisions, and that these latter are again made up of the previously known minute connective tissue fibrillæ, which are so small that their diameter can only be approximately estimated at 0.0002 to 0.0003 millim. From the connective tissue bundles of the skin of the ox, "treated by lime or baryta water, there can," he states, "be isolated from each bundle a number of component elements which have a considerably larger diameter than the minute fibres known as connective tissue fibrilla." These elements have, he remarks, in the ox a thickness of 0·003-0·006 millim., and he proposes to call them connective tissue fibres (Bindegewebsfaser). In a plate attached to his memoir the bundles and their divisions are shown in a very distinct manner.

This observation of Rollett's has not arrested the attention of anatomists to the degree which might have been expected, and seems, indeed, to have been to a great extent neglected. Two of the latest standard works may be quoted in illustration of this remark. W. Krause, in a volume on "General and Microscopic Anatomy," published in 1876, describes the tissue of the corium proper as being composed of "a network of strong bundles of connective tissue closely interwoven, the bundles being partly cylindrical, partly flattened.” There is nothing said about the subdivision of the bundles, as described by Rollett.

The same author, in his chapter on connective tissue, states, "that the ground substance of fibrous tissue consists of closely-packed, very fine, round connective tissue fibrillæ, measuring 0.0002-0·002 millim." The larger of these measurements is inapplicable to the fibrilla of Rollett, and is so near that of the subdivision or "fibre" of that author, that it is evident that Krause does not recognise the distinction between the fibre and the fibrilla established by the former histologist.

In Quain's "Anatomy "+ it is stated, "that the corium is made up of an exceedingly strong and tough framework of interlaced connective tissue fibres with blood-vessels and lymphatics. The fibres are "Sitzungsbericht der Kaiserlichen Akadamie der Wissenschaften," vol. xxx. + Eighth edition, edited by Dr. Sharpey, Dr. A. Thomson, and Mr. Schäfer; p. 213, vol. ii.