REPORTS

ON

THE STATE OF SCIENCE.

Report on the present state of our knowledge of Refractive Indices for the Standard Rays of the Solar Spectrum in different media. Presented to the British Association for the Advancement of Science. By the Rev. BADEN POWELL, M.A., F.R.S., F.G.S., F.R. Ast. S., Savilian Professor of Geometry, Oxford.

[With two Plates.]

IN submitting to the British Association a Report on this subject, on the present occasion, after having already, from time to time, made various statements relative to such results, both to the Association and in published papers,-it may be necessary to explain, that those former statements embraced only detached portions of the subject, and in many instances contained only first, imperfect, and approximate results, which I was nevertheless anxious to bring forward at the time, in order to afford some means of attempting comparisons with theory. These results have been since rendered more accurate by further repetitions, and some points which seemed doubtful, now cleared up: though I have still to regret that it has not been in my power, as yet, materially to extend the range of media examined. To state, however, the entire series of such results as I can now with any confidence offer, and to collect in one view all the other determinations of the kind as yet known to have been obtained, is the object of the present Report.

The historical memoranda of these researches may be briefly stated by way of introduction.

Fraunhofer was beyond question the first who used the dark lines as points of measurement for the deviations of definite primary rays, in seven kinds of glass, and three liquids. His original memoir appears in the Munich Transactions (DenkVOL. VIII. 1839.

B

schriften der Academie der Wissenschaften zu München für die Jahre 1814, 1815, band v.). This was translated into French in Schumacher's Astronomischen Abhandlungen, Zweites Heft, Altona 1823; and from this last an English translation was made in the Edinburgh Philosophical Journal, Nos. 18 and 19, Oct. 1823.

M. Rudberg (in Poggendorff's Annalen, band xiv. § 45, and band xvii.) gave similar series of results for the ordinary and extraordinary refractions, and those along the axes of elasticity of several crystals.

The importance of extending such observations was urged by Sir J. F. W. Herschel, in his Treatise on Light (§ 1117. and 1121.), in 1827. The same recommendation was repeated by Sir D. Brewster, in his Report on Physical Optics to the British Association in 1833 (Second Report, p. 319); and with peculiar force, as coming from a philosopher who had done so much for the determination of refractive powers, and almost everything for the dispersive powers, of a vast range of substances.

This led to the formal recommendation of such researches by the Association (Third Report, p. 473); and a grant was placed at my disposal, for the prosecution of this object, at the Dublin Meeting, 1835. I used every endeavour to carry on the observations; and though I found the difficulties in practice greater than I had anticipated, I was able to present at the Bristol Meeting in 1836, and circulated in print, a series of first approximate determinations of indices for various media, which was afterwards embodied in the memoirs of the Oxford Ashmolean Society. Through the same body I published "Additional Observations," &c. in 1838, containing repetitions of some important measurements, with a view, not only to increased accuracy, but to the settlement of some points which seemed doubtful. Some discussion which took place on these points at the Newcastle Meeting, 1838, led to a further examination; and during the present year I repeated the most important determinations, together with some additional investigations, which were printed by the Ashmolean Society under title of "A Second Supplement to Observations," &c., 1839. These are referred to in the following report under the designation of First, Second, and Third Series, respectively.

The object of this Report is to bring together all the data we at present possess of this kind, in a uniform tabular arrangement, which will comprise,

1st. Fraunhofer's Indices.

2nd. Those of M. Rudberg.

3rd. Those obtained by myself, collected from my several

papers, taking the mean where the different sets were comparable; and in other cases adopting those which appeared to me most to be relied on.

My observations were made with an apparatus, the essential parts of which are a graduated circle, having the prism at its centre, and a small achromatic telescope with cross wires, and a power of 10 nearly, directed to the prism, and moveable on an arm about the centre, along with the index. The diameter is 10 inches; the limb is divided on silver to 10', and by two opposite verniers with lenses to 10". It was originally made and divided by Allan, but fitted up for this special purpose, under my directions, by Mr. Simms. The whole will be directly understood by inspection of the annexed plate. (See Plate I.)

The slit, which is the origin of light, is about th of an inch broad, formed by the edges of two brass plates made by Mr. Simms, and inserted in a screen, outside of which is the usual apparatus for throwing the sun's rays into any convenient direction; the prism is placed at about 12 feet distance.

The absolute deviation of each ray is thus observed directly from the zero point, or that which is shown on directing the telescope to the slit. The adjustment for parallelism in the edge of the prism with the slit is easily made, and the position of least deviation found accurately by the cross wires and fixed lines.

From the observed deviations, the indices are deduced by the well-known formula (8 being the minimum deviation, the prism-angle, and μ the index,)

For liquids, hollow prisms or troughs of truly parallel plate-glass were employed, the angle of each being determined by a method described in my second paper; a thermometer was inserted, and the temperature of the medium always noted.

In the course of my observations some doubt had arisen as to the exact identification of certain of the standard rays, according to Fraunhofer's designation of them, owing to the very defective representations given of them in various optical treatises, which fail to convey the peculiar characteristics which mark the different bands.

Among the larger maps of the spectrum, that in the Edinburgh Encyclopædia (art. OpTics, plate 433, fig. 16.) is professedly copied from Fraunhofer's, which is given in Schumacher's Journal, before referred to, (tab. ii. fig. 6.); and this, taken from that in the Munich Transactions (tab. ii. fig. 5.). This last

appears to me superior in delicacy of representation, conveying by shading (which is by no means so good in Schumacher's print) an idea of the relative intensity of the different parts of the spectrum. Both preserve admirably the varied characters of the several groups of lines, and present a faithful picture of the actual object. All this, however, is almost entirely lost in the plate in the Edinburgh Encyclopædia, the execution of which is coarse, and the characters of the different bands ill preserved, especially at G; while two small groups of lines between and below the bands at H, are made so conspicuous as to be mistaken for them. In the plate in the Edinburgh Philosophical Journal (No. 18.), H is distinctly marked at the point midway between the two bands, instead of being opposite the lower.

In the plates in the Munich Transactions, and in Schumacher's Journal, the appearance of the numerous lines about G is beautifully given; and I have closely compared these representations with the actual object, both as seen in the small telescope of my apparatus, and also in one with a power of 20. With this power all the smaller lines are seen as in Fraunhofer's plate, but it is insufficient to resolve the two broad bands. that plate, however, they are represented as formed of masses of very fine lines close together; and in the less refrangible group, as nearly as possible at its centre, there is one line a little stronger than the rest, opposite to which G is marked.

In

Thus, the middle of the lower band, in my observations, appears correctly taken for the exact position of G. As some guide to the appearance and position of the lines, I have annexed a map laid down from my own observations, and giving their general character as exhibited in the small telescope of my apparatus (see Plate II.).

With regard to the accuracy of the observations, and the degree of accordance between one set and another, it should be borne in mind, that the liquid prism is necessarily exposed to the heating power of the sun's rays during the whole time of observation. Hence, the refractive power will be liable to continual small changes; for which evil no remedy seems applicable, but that which may be supplied in multiplying observations, from which, it may be presumed, the resulting mean values will furnish determinations on which increasing reliance may be placed. In general, the observations at higher temperatures are less to be depended on.

No comparison can be made between the refractive powers of different media, until some means can be found of reducing them to a common temperature. I find a proportional diminution

of the indices, for an increase of temperature will not hold good except within very confined limits.

In many media the violet and blue rays are absorbed ; and in others the lines are very faint, or invisible: in some such cases, however, their position might be estimated nearly by means of coloured glasses. Some results of this kind I have stated, distinguishing them as only rough approximations, in cases where, from the nature of the substance, we can hardly hope to obtain more accurate results. Among those media of this class which I have recently examined, is the liquid ammonia; the volatile nature of the substance being such as to occasion so great a want of homogeneity that no lines are visible, and the measures are only rough estimations.

In some cases, and those among the most interesting, not even such approximate measures appear attainable. This was especially the case in that highly important instance, the chromate of lead. A good specimen of this crystal was kindly presented to me by H. J. Brooke, Esq., F.R.S., which was (not without considerable difficulty) cut into a prism of small angle by Mr. Dollond. But the appearance of the spectrum was altogether confused, and the blue end wholly absorbed, so that no measures could be obtained.

In some of my first series of observations, no distinct measures were taken for the two bands at H, but the middle point between them was taken. Such results are rejected in the present report. But in the low-dispersive cases, the difference is so minute, that I have thought it quite sufficient to introduce a small proportional correction, to reduce the index to the exact position of H.

Some observations given from the first series, for certain chemical solutions of very low dispersive power, are of little value, since the differences of temperature render comparison impracticable, though the whole subject of the indices of chemical compounds, compared with those of their elements, is one well deserving more full investigation.

Results for a few other media are added from observations now first published; among which will be found rock-salt, so interesting from its relations to heat.

I had entertained hopes of being able before this time to obtain results for a much more extended range of substances, especially those of the more highly dispersive class. But there are many difficulties in procuring specimens in a state susceptible of this mode of prismatic examination; there are also many substances, and among them the most important, to which it is to be feared such examination cannot be applied.