556 
ASTRONOMY: F. H. SHARES 
Keeler, along the southern boundary of the Huyghenian Region, and 
to show that the color was probably due to the Ha line of hydrogen. 
Spectrum photographs by Adams subsequently placed the matter 
beyond doubt. ^ 
The usefulness of monochromatic photographs seems thus to have 
been demonstrated, but it does not appear that they have been employed 
except for the Orion Nebula. A wider application has therefore seemed 
desirable, particularly for objects too faint for detailed spectroscopic 
observation. Among these one naturally turns to the spiral nebulae 
because of the unexpected result uniformly found for the central region, 
namely, an absorption spectrum of intermediate or late type. Even 
admitting the presence of superimposed bright lines, which has in some 
Iso 
32™ 
16™ 
gm 
4m 
2m 
Seed 23 
gm 
4m 2™ 1™ 30S 
FIG. 4. N. G. G. 3242. 
cases been claimed, the result is still remarkable for objects whose finer 
details are suggestive of some of the gaseous nebulae. 
The illustrations show the distribution of the blue and yellow light 
in three of the well-known spirals, Messier 51, 94, and 99. The photo- 
graphs were made with the 60-inch reflector on Seed's '27' and Cramer's 
'Instantaneous Isochromatic' plates, the latter exposed behind a yellow 
filter. The exposure time for yellow light was six times that for blue, 
in order that the images for the bluer stars might be comparable on the 
two photographs. 
Few if any of the yellow images are smaller than the corresponding 
blue images, at least for objects which are certainly stars; but this 
is by no means the case for the condensations and nebulosity comprising 
the branches of the spirals. For these the relative weakness of the 
