518 
ASTRONOMY: F. G. PEASE 
that their spectra do not overlap. For the comparison spectrum an- 
other silvered glass plate is made in which for each continuous cut in 
the slit-plate there is an interrupted cut (fig. 2), the central portion 
covering the nebula while the comparison exposure is made on a spark 
between iron electrodes. This second plate is mounted on a metal frame 
which swings it almost into contact with the slit-plate when the spark 
is used. When not in use it is turned out of the way. A dense piece of 
opal glass between the spark and the slit serves to diffuse the light 
thoroughly. No projecting lens is employed. It is of course neces- 
sary in this arrangement to place the photographic plate at right angles 
to the axis of the camera. 
At the Seventeenth Meeting of the American Astronomical Society, 
Slipher*^ gave for the nebula N. G. C. 4594 a radial velocity of + 1100 
km. per sec, and stated that it has a rotation analogous to that of the 
planets. He assigns a value of 100 km. per sec. to the linear velocity of 
rotation at a distance of 20 seconds of arc from the nucleus. 
FIG. 1. FIG. 2. 
A direct photograph (fig. 3) of this nebula shows it to be a spiral 
seen almost edge on, with the convolutions so nearly concentric that it 
is not possible to state whether it is of the right or left-handed type. The 
nebula is crossed by a dark streak which appears to absorb or obstruct 
the light. This streak seems to lie at the periphery, being possibly an 
outer ring of cooler material, or perhaps the unilluminated edge of 
the thin disk of nebulous matter surrounding the brilliant nucleus. 
With possible differences of temperature in mind, a photograph was 
taken on a red sensitive plate through a color screen transmitting the 
region X 5650-7600, to test for changes of contrast in visual and photo- 
graphic light. No certain difference was found between this photo- 
graph and one taken upon a Seed '23' plate. 
A spectrum exposure of 80 hours was made at intervals during March, 
April, and May, the slit being placed along the major axis of the spindle 
across the nucleus. Since the nebula is long and narrow a single straight 
line was in this instance cut in the slit-plate, while the spark plate was 
