ASTRONOMY: H. D. CURTIS 
11 
of these remarkable objects, so they are also inadequate as records of 
position for the determination of proper motions and rotations. Trust- 
worthy results can be secured only by the aid of photography. 
Sixteen years have now elapsed since Keeler inaugurated his pro- 
gram of nebular photography with the Crossley Reflector of the Lick 
Observatory; and Keeler's plates, in connection with recent photographs 
secured with the same instrument, afford a large mass of material for the 
investigation of nebular motions. Some of the diffuse objects of the 
Keeler program do not admit of precise measurement; some also of the 
old plates are rather poor; but when the program of repetition, com- 
menced early in 1914, shall be completed, in 1915, sets of photographs, 
early and recent, with an average interval of time of about thirteen 
years, will be available for the determination of the proper motions or 
internal movements and changes of between 80 and 100 nebulae. 
This program is now about one- third complete, as regards the taking 
of the modern plates, and about one-fourth of the measurements have 
been made. The work to date comprises most of the larger spirals, such 
diffuse nebulosities a's the Orion and Network nebulae, several planetaries, 
and several very faint small nebulae 
As the early and late plates of this investigation have been secured 
with the same instrument, the methods of measurement have been made 
entirely differential. Five or six symmetrically-placed small stars are 
selected as a common reference system for all the plates of a given nebula, 
and the nucleus and numerous knots or condensations are measured in 
each nebula as available. 
Detailed results will be pubHshed upon the completion of the program, 
but from the observations already made we may draw the following 
general conclusions, which will probably not be seriously modified by 
the inclusion of more objects : 
1. In this average interval of thirteen years it has not been possible 
to detect any evidence of internal movement, rotatory or otherwise, in 
the nebulae measured. 
2. The derived proper motions are so small that they may well be 
ascribed very largely to the difficulty and uncertainty of measuring these 
difficult objects. The largest proper motions foimd are for the planetary 
nebulae N. G. C. 6905 and 7009: 0.056'' and 0.054'' per year, respectively. 
Those for the other objects measured will average two or three hundredths 
of a second of arc per year. As the plates are exposed from two to four 
hours, and as the nebular condensations are much less sharp than star 
images, the errors of measurement are manifestly much larger than they 
would be in the case of short-exposure stellar images. 
