slit; but if I commence operations by allowing the light to come first through one small portion of the slit, then we shall get the light from the particular metal which I employ in the electric arc falling on one part of the plate, and registering itself on the photographic plate. Then, if I close up that part of the slit, and open another one, I shall be able, through that newly opened part of the slit, all the rest being closed, to photograph on the plate the spectrum of another substance; say iron. Then, having used up that part of the plate, I can close that portion of the slit, I can bring my window lower down, and there obtain, say the spectrum of cobalt. The window is next brought farther down, for -say-the spectrum of nickel, so that we have, as the work of some eight or nine minutes at the outside, a photograph which will register with the most absolute and complete accuracy and certainty hundreds of lines. Now a careful student of those lines, working as hard as he can, thinks himself very fortunate if he can lay down ten an hour. Therefore, as ten in an hour is to hundreds in seven minutes, so is the eye to photography in these matters.

§ 8. My Arrangements.

The spectroscope employed contains three prisms of 45° and one of 60°; its observing telescope is replaced by a camera with a quartz lens of 2-in. aperture, of about 5-feet focal length. With this arrangement-the spectrum being received upon a sensitized plate—the por

tion between the wave-lengths 39,00 and 45,00, can be obtained at once in good focus. A ray of sunlight, reflected from ́a heliostat mirror so as to fall upon the slit-plate, is brought to a focus, by means of a double convex lens, just between the carbon poles of an electric lamp, while a second convex lens, placed between the lamp and the collimator tube, serves to cast an image of the sun or of the electric arc upon the slit-plate. Supposing, now, we wish to compare the iron spectrum with that of the sun: the sun's image in sharp focus on the slit-plate is first allowed to imprint its spectrum on the prepared plate. The ray of sunlight is then cut off, the sliding plate moved up or down till the pin catches in the next notch, and the image of the arc, between an upper pole of carbon and a lower pole consisting of a carbon crucible containing a fragment of iron, is allowed to fall on the portion of the slit thus exposed.

The following details will render the application of the method quite clear :-The laboratory in which the work has been carried on has two windows, one nearly (magnetic) south, the other nearly (magnetic) west. Outside each window level slate slabs have been erected as supports for a heliostat. Either window can be used at pleasure. The spectroscope is supported on a platform on rollers, the height of the platform being such that the horizontal beam from the heliostat is coincident with the axis of the collimator. In addition to the lens placed between the lamp and the slit to throw an image of the arc on the latter, another lens is introduced

MIXN

FIG. 35.-Another view of the spectrum photographic arrangements, showing camera.

between the heliostat and the lamp-heliostat, lenses, lamp, and collimator being of course in the same straight line. The action of the newly interpolated lens is to throw an image of the sun between the poles of the lamp, so that when the spectrum of the arc is properly focused by the camera lens on to the photographic plate, the solar spectrum, when subsequently thrown in, is also in focus.

By these means we not only obtain a photographic record of the long and short lines with the individuality of each, but we get the solar spectrum as a scale.

The accompanying diagrams (Figs. 36-38) show the arrangements adopted in the cases mentioned.

In order to obtain photographs of the solar spectrum of the very best kind, it is necessary to limit the beam passing through the prisms to very small dimensionsa method employed with such admirable results by Mr. Rutherfurd.

In the attempts to photograph the long and short lines of metallic spectra, it was found that this object could not be well obtained with the electric lamp in its usual position (with vertical poles), as the central column of dense vapour, as a rule, extended across the arc, i.e. from pole to pole, and gave all the short lines.

In order to obviate this a horizontal arc is used. This is accomplished by placing the lamp on its side and firmly securing it in that position. The image of the horizontal arc is then thrown on the vertical slit as described in Chapter II. This method is found to be perfectly successful. The central portion of the spectrum

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