October 1, 1915] 



SCIENCE 



443 



from the pole of the galaxy to the galaxy 

 itself. The importance of this fact is that 

 it shows a close connection between the 

 Milky Way and the stars nearer to us. The 

 Milky "Way is not a system of stars beyond 

 the others, but is the primary feature of our 

 ' ' island universe. ' ' 



Photometric observations have acquired 

 additional importance from the differences 

 between photographic and visual magni- 

 tudes. The ordinary plate is more sensi- 

 tive to blue light than the eye, and the 

 difference between the photographic and 

 visual (or photo-visual) magnitude of a star 

 is an index of the color. The color index 

 is found by observation to be related very 

 closely to the type of spectrum. Professor 

 Scares has shown from the color indices 

 that as the stars become fainter they be- 

 come progressively redder. Professor 

 Hertzsprung has found the same thing by 

 the use of a grating in front of the object 

 glass. Among stars of 17.0 m. visual mag- 

 nitude, Scares found none with a color 

 index less than 0.7 ; this is approximately 

 the color index of a star of solar type, i. e., 

 near the middle of the range from blue stars 

 to red stars. 



There are three ways in which this may 

 occur. The stars may be bright but very 

 distant red stars ; or they may be faint red 

 stars, like those in the immediate neighbor- 

 hood of the sun ; or there may have been an 

 absorption of blue light. It is not possible 

 to say in what proportion these causes have 

 contributed. The red stars of 9.0 m. and 

 10.0 m. are nearly all very luminous but 

 distant bodies, but it seems likely that stars 

 of 17.0 m. will contain a greater proportion 

 of stars of small luminosity. 



The absorption of light in space is very 

 small, and as yet imperfectly determined. 

 Professor Kapteyn and Mr. Jones, by com- 

 paring the color indices of stars of large and 

 small proper motion, make the difference 



between the absorption of photographic and 

 visual light as 1 m. in 2,000 parsecs. The 

 question has been examined directly by 

 Professor Adams, who has obtained spectra 

 of near and distant stars which are identical 

 as regards their lines, and has examined the 

 distribution of the continuous light. This 

 direct method of comparison showed that 

 the more distant star was always weaker in 

 violet light. But as both these investiga- 

 tions show that very luminous stars are in- 

 trinsically somewhat bluer than less lu- 

 minous stars of the same spectral type, the 

 two causes require further research for 

 their disentanglement. The question is of 

 importance, as it may serve in some cases 

 to determine the distances of very remote 

 bodies the type of spectrum of which is 

 known. 



It must be admitted that we are as yet 

 very ignorant of the more distant parts of 

 the "island universe." For example, we 

 can make little more than guesses at the 

 distance of the Milky Way, or say what 

 part is nearest to us, what are its move- 

 ments, and so on. But, nevertheless, the 

 whole subject of the construction of the 

 heavens has been opened up in a remark- 

 able manner in the last few years. The 

 methods now employed seem competent to 

 produce a tolerably good model showing the 

 coordinates and velocities of the stars as 

 well as their effective temperatures and the 

 amount of light they radiate. Industry in 

 the collection of accurate data is required, 

 along with constant attempts to interpret 

 them as they are collected. The more accu- 

 rate and detailed our knowledge of the 

 stellar system as it is now, the better will be 

 our position for the dynamical and physical 

 study of its history and evolution. 



F. W. Dyson 



The Eoyal Obsebvatoky, 

 Greenwich 



