The instrument used by MM. Bunsen and Kirchhoff, though more complicated, is constructed on the same principle as the preceding. A sunbeam transmitted by a very narrow vertical slit passes through four prisms, which disperse it so much, that if drawn on the scale seen with the magnifying telescope which receives it, the spectrum would extend over twenty feet. By means of a micrometer screw, the telescope can be turned round a vertical axis, and as the dark lines come successively under the cross wires in its eye-glass they are seen to pass over a graduated scale, so that the distances between two thousand of them have been measured in millimetres with unerring accuracy, but that is only a small part of the whole. When viewed through the telescope, the retina of the eye is the screen on which this wonderful spectrum falls, crossed by innumerable dark rayless lines of various breadths and intensities. Black bands given by the inferior refraction of one prism are here resolved into numerous dark lines as fine as a spider’s thread. Mr. Glaisher during his tenth scientific balloon ascent devoted his attention for a time almost entirely to the dark lines on the solar spectrum. At a height of about four miles and a half, they were almost innumerable; all he had seen on the earth were there, and many more. The nebulous lines H were both seen, the spectrum was a good deal lengthened at the violet end, and at the red end the line A was visible. The light from the sky near the sun gave a shorter spectrum; the lines were only visible from B to G. Besides these cosmical or permanent lines, Sir David Brewster observed that certain dark bands and lines in the red and green parts of the spectrum are only visible when the sun is near the horizon, whence he concluded that they are occasioned by the absorption of the solar light while traversing a thicker stratum of air than when the sun is in the zenith. Various groups of these absorption bands are to be seen at times on the solar spectrum, especially a remarkable one near Fraunhofer’s line D, and Dr. Miller observed that temporary dark lines appeared during a heavy shower, which vanished when the rain ceased. When the sun was high, M. Kirchhoff mentions that he had noticed traces of lines and nebulous bands in different parts of the spectrum, which he thinks might be resolved by a greater number of prisms than those in his apparatus.