1 6 POPULAR SCIENCE MONTHLY. 



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 pho- 

 tosphere (Fig. 7). A sharp distinction must, however, be drawn be- 

 tween this appearance, which is wholly invisible to the eye at the tele- 

 scope, 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 antici- 

 pated 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 faculae, 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 faculas 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 



