10 
ASTRONOMY: H. D. CURTIS 
or new stars^ — stars which suddenly flash up where previously no stars, 
or at the best, very faint stars, were known to exist — is that they are 
due to the rushing of dark or relatively faint stars through invisible 
nebulae or resisting media. The resulting colHsions, in cEect a bombard- 
ment of the stellar surfaces, seem to generate sufficient heat to convert the 
surface strata of the stars into incandescent gases and vapors. Spectro- 
graphic observations have shown for all recent novae that in the course 
of a few weeks or months the spectra of the novae are converted into the 
nebular type. Later the nebular spectra disappear and a certain type 
of stellar spectrum takes their place. These changes occur rapidly, 
perhaps because the bombardment effects have been but skin deep. 
That planetary nebulae may have resulted in some such way from the 
collisions of stars and the resisting media seems quite possible. Exactly 
those stars which are travelling with very high speeds would have the 
greatest chance to encounter resisting media: and further, the disturb- 
ances would be deeper and more permanent the higher the speed of 
collision. Conversely, if this speculation has a basis in fact, that is, 
if the planetary nebulae have been formed in this manner, the pre- 
vaihng high velocities would find a natural explanation. 
In securing the difficult long-exposure observations upon which this 
note is based, I have been greatly assisted by several members of our 
staff, especially by J. H. Moore, E. S. Haynes, and P. W. Merrill. 
PRELIMINARY NOTE ON NEBULAR PROPER MOTIONS 
By H. D. Curtis 
UCK OBSERVATORY. UNIVERSITY OF CALIFORNIA 
Presented to the Academy, October 31.1914 
The great number of nebulae in the sky, their well-known grouping in 
regions distant from the Milky Way, the spiral structure so frequently 
occurring, and the question as to the place of these bodies in the evolution 
of the sidereal universe, all present a series of problems as yet only 
partially solved. 
A knowledge of the proper motions or of any rotational movements 
which these bodies may have would be of very great value in investiga- 
tions as to the size and distance of the nebulae, and therefore as to their 
place in the structure of the visible universe. Owing to the hazy and 
diffuse character of the great majority of nebulae when observed visually, 
the visual observations of the past, just as they have proved nearly 
valueless in giving an adequate conception of the form and structure 
