ASTRONOMY: F. H. SHARES 
555 
Keeler^ to confirm Campbell's results for the nebula in Orion. It shows 
admirably, but without differentiation, the distribution of whatever 
luminosity lies between the limiting wave-lengths determined by the 
filter and the color curve of the plate. Some refinement of analysis 
can be effected by using filters of limited transmissibility. Thus Hart- 
Iso ih 
FIG. 2. MESSIER 94. 
Seed 27 20"^ Iso V 
FIG. 3. MESSIER 99. 
mann^ showed that the great photographic activity noted by Keeler 
in certain regions of the Orion nebula is due to X 3727, whereas the wider 
spectral region embraced in Keeler's photographs left the identity of the 
active radiation in doubt. Again, a red-sensitive plate and a filter 
transmitting nothing to the violet of X5600 enabled Hale^ to demonstrate 
the reality of the reddish fringe observed by Barnard, and later by 
