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of the above range, more especially for any of which the period is between 24 and 25 days. We find strong evidence of an inequality of considerable magnitude of which the period is 24-00 days, very nearly. We have also found preliminary evidence of the existence of two considerable inequalites of periods not very far from 24.65 and 24.80 days. These two appear to come together in about 11 years, but we cannot yet give the exact time of this.
We have not found a trace of any inequality with a period of 24-25 days.
V. "Some Experiments on Metallic Reflexion." By Sir JOHN CONROY, Bart., M.A. Communicated by Professor G. G. STOKES, Sec. R.S. Received November 18, 1878.
In the experiments made by Sir David Brewster, M. Jamin, Professor Haughton, and others, on the light reflected by polished metallic surfaces, the reflecting surfaces were in contact with air; and, as far as I am aware, the only observations which have been made when the reflecting surfaces were in contact with other media are those by Quincke, an account of which is given in "Poggendorff's Annalen,' vol. cxxviii, p. 541, and in the "Jubelband," p. 336. He found that he obtained different values for the principal incidence and principal azimuth, according as the reflecting surface of a film of silver was in contact with air, crown glass, flint glass, water, or turpentine, and that the only connexion between the values of these angles and the refractive index of the medium in which the reflexion took place was, that in general with the same metal, the principal incidence and the principal azimuth became less as the refractive index of the medium increased.
I therefore hope that a short account of some attempts that I have recently made to determine the principal incidence for, and the principal azimuth of, the light reflected by polished surfaces of gold and in contact with different media, may be of interest.
The experiments are, I regret to say, incomplete, as, finding that my eyes were beginning to suffer, I thought it best, for the present at least, to discontinue them.
I used a Babinet's goniometer, to the arms of which two tubes containing nicols were attached, a vertical divided circle being fixed at one end of each, so that the position of the nicols could be read by a vernier to 5'. The goniometer had, in addition to the horizontal stage, a vertical one, so arranged that the reflecting surface could be placed in the axis of the instrument; toothed wheels, working into a pinion rotating on an axis fixed in one of the arms of the divided circle, were attached to the vertical stage, the position of which could
be read by a vernier to 15", and to the telescope arm; the ratio of the wheels to the pinion being such that, on moving the telescope arm, the vertical stage also moved in the same direction, but with half the angular velocity; so that when the reflecting surface had been properly adjusted, the light which passed along the axis of the tube fixed to the collimator arm, was reflected along the axis of that fixed to the telescope arm, at all angles of incidence.
A quarter undulation plate was placed at the inner end of the tube fixed to the collimator arm, and a small direct vision spectroscope, with a photographic scale, could be attached to the other tube. The lower edge of the vertical stage being some distance above the graduated circle of the goniometer, a cylindrical vessel of thin glass, about 6 centims. in diameter, could be placed on the horizontal stage, so as to surround the lower part of the reflecting surface; this being filled with the liquid, and a narrow vertical slit placed so as to limit the incident light, fairly good observations could be made when the reflecting surface was in contact with various liquids.
When the principal section of the first or polarizing nicol was inclined at an angle of 45° to the plane of incidence, and one of the neutral axes of the quarter undulation plate placed in that plane, the transmitted light was elliptically polarized; and at a particular incidence, varying with its refrangibility, it was reflected by the metallic surface as plane polarized light; the plane of polarization being determined by the second nicol.
Had the retarding plate really been "a quarter undulation plate" for light of any given wave-length, the angle of incidence at which it was reflected as plane polarized light, and the azimuth of its plane of polarization, would have been the principal incidence and principal azimuth for light of that refrangibility.
The retardation of a given plate varies so much for different portions of the spectrum, that even had it been possible to obtain one producing a retardation of exactly 90° for light of any definite refrangibility, it would have differed greatly from a quarter plate for other portions of the spectrum.
Both the neutral axes of the plate were successively placed in the plane of incidence, and the mean of the two values of the angle of incidence taken as the principal incidence.
This arrangement is very similar to the one used by Dr. Eilhard Wiedemann in his observations on the light reflected by surfaces of fuchsine and copper, and described in “Pogg. Ann.,” vol. cli, p. 6. In Dr. Wiedemann's experiments the angle of incidence remained constant, the position of the quarter undulation plate and of the nicol being varied; whilst in mine, the position of the quarter undulation plate was constant, and the angle of incidence and the position of the nicol were altered. By this means the principal incidence and azimuth
were determined directly, but less accurately than by Dr. Wiede
mann's arrangement, which, however, involves a good deal of cal
The analysing portion of Dr. Wiedemann's instrument appears to differ merely by the addition of a small direct vision spectroscope from the Elliptic Analyser of Professor Stokes, described in the "Report of the British Association for 1851," Part II, p. 14.
The experiments were made with sun-light and with lamp-light; with the former, the angle of incidence and the azimuth of the analysing nicol were altered till the dark band in the spectrum was most intense at certain definite positions, as measured by the scale of the spectroscope; with the latter, till the light which had passed through a piece of red glass was reduced to a minimum.
Numerous measurements were made of both these angles with a plate of gold in air, water, carbon bisulphide, and carbon tetrachloride; and of copper, in air, water, and carbon tetrachloride; but the results were not very satisfactory. In addition to the difficulty of determining accurately the zero of the nicols, and of placing the neutral axis of the quarter undulation plate in the plane of incidence, I found that very different values were obtained for the principal incidence, according as one or other of the neutral axes of the quarter undulation plate I was using was in the plane of incidence.
In all cases, however, the principal incidence which, as is well known, is less for the more refrangible rays, diminishes, and the principal azimuth increases with the increase of the refractive index of the medium in contact with the metallic surface; and further, the diminution in the value of the principal incidence appears to be nearly in proportion to the increase of the refractive index of the surrounding medium.
The decrease of the principal incidence, with an increase in the refractive index of the surrounding medium, is exactly what might be expected to take place if the principal incidence for a metal were the same as the angle of polarization of a transparent substance; that is, the angle whose tangent is equal to the refractive index.
If such is the case, the metals must all have very high refractive indices; but some experiments of Quincke's ("Pogg. Ann.," vol. cxix, p. 379, and vol. cxx, p. 602) appear to show that their refractive indices are less than 1.
The following are some results I obtained with a gold plate (formed by soldering a slip of thin sheet gold to a brass plate), in air, with lamp-light, a deep red glass being interposed; the position of the quarter undulation plate in which the ray polarized perpendicularly to the plane of incidence was retarded relatively to the other, being called A, and that in which the retarded ray was the one polarized in the plane of incidence, B. The signs of the azimuth of the plane of
polarization of the reflected light show which ray is retarded by the plate; and, to confirm this, the light transmitted by the nicol and plate was examined with an Iceland spar, cut to show the rings.
The azimuths are reckoned as positive when measured from the plane of incidence in the direction in which the hands of a watch move, to a person supposed to be so placed as to receive the light, whether incident or reflected, into his eye.
Similar measurements were made when the gold plate was in water and carbon bisulphide. The values of the incidences differed greatly according as one or other of the neutral axes of the quarter undulation plate was in the plane of incidence, the measurements being about as concordant as those made with the gold plate in air; the means of these determinations were taken as nearest the truth.
Since the retardation of the ray polarized perpendicularly to the
plane of incidence probably varies more for each degree when the light is incident at an angle greater than that of the principal incidence, than when it falls on the surface at a less angle, the mean of these two sets of determinations can only be considered as an approximation to the truth, especially when, as in this instance, the difference between the two values is a considerable one.
Mean value, from eight observations, four with the P. S. of the polarising nicol inclined to the right and four with it to the left of the plane of incidence, of the principal incidence and principal azimuth, for red light, with Quarter Undulation Plate 1.
Gold in air....
Gold in water.
Gold in carbon bisulphide.
Plate at A...
Plate at A..
Plate at A..
Plate at A..
Plate at A... 35 52
76 10 62 44 Mean value. 69 27
Plate at A....
Plate at A....
The principal incidence and principal azimuth for gold in air, with red light, were determined with six other quarter undulation plates with the following results; the numbers for Plate 1 being the mean of eight observations, whilst those of the remainder are the mean of two only, made with the polarizing nicol on either side of the plane of incidence: