MOUNT WILSON SOLAR OBSERVATORY. I95 



comprise a considerably larger number of members than were found by 

 that investigation. 



(7) The curious fact was brought to light that all the somewhat extensive 

 local groups of stars, for which the required data are known, move in space 

 very nearly parallel to the Milky Way, 



(8) Little was done on the study of the absorption of light in space. A 

 few words may be said on the subject, however, in order to remove an 

 apparently very general misapprehension. It can not be maintained that 

 the existence of an appreciable absorption of light in interstellar space has 

 been indubitably proved by the work done up to the present time. On the 

 other hand, however, the investigations published and unpublished prove 

 beyond a doubt that, in general, stars of the same spectral class and of equal 

 apparent magnitude are redder the greater their distance from our system. 

 This fact may be due either to absorption in space or to the influence of 

 the absolute luminosity. For the purpose of deciding which of the two 

 factors is the one really responsible for the observed phenomenon, the fit- 

 test instrument would seem to be a fairly powerful short-focus refractor 

 with a large field, provided with an objective prism. As this Observatory 

 will probably possess such an instrument in the near future, it seems better 

 to delay the necessary observations until it becomes available. Meanwhile 

 the fact that, other things being equal, the more distant stars are redder, 

 must not be overlooked, as it is of extreme importance to astronomy, inde- 

 pendently of the question as to which of the two factors is its cause; for it 

 will be a criterion, and possibly even an approximate measure of parallax at 

 distances where all other measures fail. 



The work of Mr. Fath during the past year on the spectra of stars of 

 the same magnitude and type of spectrum but of widely different proper- 

 motions has, on the whole, helped toward the establishment of this fact. 

 It is also worthy of note that certain stars classified by Mr. Kohlschiitter, 

 and found to be peculiarly faint in the violet portion of the spectrum, all 

 have extremely small proper-motions. 



Some time has again been devoted this year to work on the observing 

 program of the 60-inch reflector. Last year a large number of B-type stars, 

 especially such as were suspected of belonging to what is now generally 

 called the second star-stream, were put on the program. In addition a 

 number of A-type stars in both streams were included, the selection being 

 made in such a way as to insure a good determination of the stream veloci- 

 ties. Finally, a few A-type stars suspected of belonging to the Perseus and 

 Scorpius-Centaurus groups were added, together with the stars already re- 

 ferred to of large and small proper-motions, whose radial velocities, taken 

 in conjunction with the other data, might furnish some evidence as to the 

 redness of the distant stars. 



In arranging the program for the coming year, obviously the best plan 

 was to give especial attention to such stars as the preliminary investigation 



