the spectrum may be made to fall upon this slit by properly adjusting the mirror and prisms. Above the slit, and nearly in contact with it, the photographic plate is mounted in a carriage which runs on tracks at right angles to the length of the slit. The tracks are covered by a light-tight camera box, so that no light can reach the plate except that which passes through the second slit. While the solar image is moving across the first slit, the plate is moved at the same rate across the second slit, by a shaft leading clown the tube from the electric motor, and connected, by means of belting, with screws that drive the plate-carriage.
Photographs of the solar disk taken with this instrument under good atmospheric conditions show a multiplicity of fine details of which no trace appears on the Kenwood plates (Fig. 6). The entire surface of the sun is shown by these plates to be dotted over with minute luminous clouds of calcium vapor, separated by dark spaces, and closely resembling in appearance the well-known granulation of the solar photosphere (Fig. 7). A sharp distinction must, however, be drawn between this appearance, which is wholly invisible to the eye at the telescope, and the granulation of the photosphere. In accordance with Langley's view the grains into which the surface of the sun is resolved under good conditions of visual observation are the extremities of columns of vapor rising from the sun's interior. They seem to mark the regions at which convection currents, proceeding from within the sun, bring up highly heated vapors to a height where the temperature becomes low enough to permit them to condense. It might be anticipated that out of the summits of these condensed columns, other vapors, less easily condensed, might continue to rise, and that the granulated appearance obtained with the spectroheliograph may represent the calcium clouds at the summits of these columns. We might indeed go a step further, and imagine the larger and higher calcium clouds to be constituted of similar vaporous columns, passing upward through the chromosphere and perhaps at times extending into the prominences themselves. But without a means of research now to be described, which represents another application of the spectroheliograph, involving a new principle, the true nature of these phenomena could not be ascertained.
Mention has already been made of the luminous faculæ, which are simply regions in the photosphere that rise above the ordinary level. At the edge of the sun their summits lie above the lower and denser part of that absorbing atmosphere which so greatly reduces the sun's light near the limb, and in this region the faculæ may be seen visually. At times they may be traced to considerable distances from the sun's limb, but as a rule they are inconspicuous or wholly invisible toward the central part of the solar disk. The Kenwood experiments had shown