280 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
At each corner of the platform was screwed a small cast-iron block, 
in which a V-shaped groove had been planed. In each groove was 
a steel ball. A moving platform, also built of wood, carried the 
optical parts of the spectroheliograph and rested on these balls, so 
that it could be moved across the image of the sun (formed by a 
ccelostat telescope). The motion was produced by a small electric 
motor, belted with a piece of fish line to this large wooden pulley, 
which drove a screw passing through a lead nut fastened to the 
movable platform. The screw was cut on a foot lathe and the nut 
cast on it. This simple mechanism provided the means of producing 
a slow uniform motion of this upper platform across the image of 
the sun. The arrangement of the optical parts was precisely the same 
as in the Rumford spectroheliograph. 
Looking at the instrument in plan, we have a slit here (a) through 
which the ight passes. A very simple slit will do. This was an old 
one; I think it came from a portion of the old Kenwood spectrohelio- 
graph. The light passed through this slit and fell on a collimating 
lens (6), which may be an ordinary uncorrected lens if the focal 
length is sufficient. We happened to have some achromatics which 
we used, but they were no better than a simple lens would be. The 
parallel rays fell on a plane mirror here (c), and were reflected to 
these prisms (d, 7). We used two prisms, but one will do perfectly 
well, unless hydrogen as well as calcium flocculi are to be photo- 
graphed. These prisms had been discarded; they were originally 
made for the Bruce spectrograph, but they were so poor that they 
could not be advantageously used for stellar spectra, so we borrowed 
them from the Yerkes Observatory and put them in here. The two 
prisms, with the mirror, gave a total deviation of 180°. The hght 
then passed through the camera lens (e)—here, also, a simple lens will 
serve very well—which formed an image of the spectrum on a 
second slit (7), close to the fixed photographic plate (g). By set- 
ting this slit on the H, line of calcium, and moving the instrument 
slowly across the solar image with the motor, excellent photographs 
of the calcium flocculi were obtained. 
The next slide shows some photographs taken with the permanent 
instrument. Such photographs as these, made with the calcium and 
hydrogen lines, open up for investigation a large field, which anyone 
can enter with just such an equipment as I have described—a very 
simple instrument, with small prisms and lenses, and built almost 
entirely of wood. 
IT will show you in the next photograph some pictures obtained 
with the wooden instrument. You will notice that in this case the 
motion was not absolutely uniform; you can detect the slight irregu- 
larity of motion, but it did not affect the usefulness of the negatives. 
This is a direct photograph of the sun; this is made with the H, line 
