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to meet the requirements of the case. In place of the small prism, two pieces of silvered glass were securely fixed before the slit at an angle of 45°. In a direction at right angles to that of the slit, an opening of about inch was left between the pieces of glass for the passage of the pencils from the object-glass. By means of this arrangement the spectrum of a star is seen accompanied by two spectra of comparison, one appearing above, and the other below it. As the reflecting surfaces are about 0-5 inch from the slit, and the rays from the spark are divergent, the light reflected from the pieces of glass will have encroached upon the pencils from the object-glass by the time they reach the slit, and the upper and lower spectra of comparison will appear to overlap to a small extent the spectrum formed by the light from the object-glass. This condition of things is of great assistance to the eye in forming a judgment as to the absolute coincidence or otherwise of lines. For the purpose of avoiding some inconveniences which would arise from glass of the ordinary thickness, pieces of the thin glass used for the covers of microscopic objects were carefully selected, and these were silvered by floating them upon the surface of a silvering solution. In order to ensure that the induction-spark should always preserve the same position relatively to the mirror, a piece of sheet gutta percha was fixed above the silvered glass; in the plate of gutta percha, at the proper place, a small hole was made of about inch in diameter. The ebonite clamp containing the electrodes is so fixed as to permit the point of separation of these to be adjusted exactly over the small hole in the gutta percha. The adjustment of the parts of the apparatus was made by closing the end of the adapting-tube, by which the apparatus is attached to the telescope, with a diaphragm with a small central hole, before which a spirit-lamp was placed. When the lines from the induction-spark, in the two spectra of comparison, were seen to overlap exactly, for a short distance, the lines of sodium from the light of the lamp, the adjustment was considered perfect. The accuracy of adjustment has been confirmed by the exact coincidence of the three lines of magnesium with the component lines of b in the spectrum of the moon.
In some cases the spectra produced by the spark are inconveniently bright for comparison with those of the stars and nebulae. If the spark is reduced in power below a certain point, many of the lines are not then well developed; the plan, therefore, was adopted of diminishing the brightness of the spectrum, by a wedge of neutral-tint glass which can be moved at pleasure between the plate of gutta percha and the silvered mirror.
Two eyepieces were employed with the apparatus, the one magnifying four diameters, and the other six diameters.
The induction-coil was the same which I employed in my former observations. It was excited by a form of bichromate-of-potash battery, which I have found so exceedingly convenient for the occasional work of celestial observations that I will describe it here.
The battery, which was made for me by Mr. Ladd, consists of two large cells of ebonite, each of which contains two plates of graphite, 6 inches by inches, connected
together with copper bands. These plates are fixed into varnished plates of wood, which form covers to the cells. In each wooden cover is a slit of the length of the width of the carbon plates, by which an amalgamated zinc plate can be inserted between the plates of graphite. An important part of the arrangement is a third large cell of ebonite, which is filled with water acidulated with a few drops of sulphuric acid, and contains at the bottom some mercury. As soon as a plate is removed from the battery after use, it is rubbed clean under running water, and then immersed in the spare cell until it is again required. By this arrangement the plates are always clean and perfectly amalgamated. The solution employed to charge the battery is a saturated solution of bichromate of potash, to which about part of sulphuric acid has been added. The battery is sufficiently powerful and always ready for instant use. For the convenience of varying the power of the battery, three sets of zinc plates are kept in the spare cell; the plates are
2 inches, 4 inches, and 6 inches in width.
§ III. Observations of Nebulas.
For the greater convenience of reference and of comparison, the spectrum of 37 H. IV. Draconis from my paper “ On the Spectra of some of the Nebulae”*, has been added to fig. 2, Plate XXXIII. The spectrum of this nebula may be taken as characteristic, in its general features, of the spectra of all the nebulae which do not give a continuous spectrum. At present I have determined satisfactorily the general characters of the spectra of about seventy nebulae. This number forms but a part of the much larger list of nebulae which I have examined, but in the case of many of these objects their light was found to be too feeble for a satisfactory analysis. Of these seventy nebulae about one-third give a spectrum of bright lines. The proportion, which is indicated by this examination, of the nebulae which give a spectrum of bright lines to those of which the spectrum is continuous (namely, as one to two), is probably higher than would result from a wider observation of the objects contained in such catalogues as those of Sir John Hersciiel and Dr. D’Arrest, since many of the objects which I examined were specially selected, on account of the probability (which was suggested by their form or colour) that they were gaseous in constitution.
All the differences which I have hitherto observed between the spectra of the gaseous nebulae may be regarded as modifications only of the typical form of spectrum which is represented in the diagram, since they consist of differences of relative intensity, of the deficiency of one or two lines, or of the presence of one or two additional lines. It is worthy of remark that, so far as the nebulae have been examined, the brightest of the three lines, which agrees in position in the spectrum •jvith the brightest of the lines of the spectrum of nitrogen, is present in all the nebulae which give a spectrum indicative of gaseity. It is a suggestive fact that should not be overlooked, that in no nebula which has a spectrum of bright lines, has any additional line been observed on the less refrangible and brighter side of the line common to all the gaseous nebulae.
* Philosophical Transactions, 1864, p. 438.