I9I2-] DEPTH OF THE MILKY WAY. 6 



remoter stars of the Alilky Way by Newcomb and many other 

 modern writers ; but of course it is much too small, for the follow- 

 ing reasons : 



(a) Newcomb and other astronomers cite the possibility of 

 some of the stars being as much as 1,000,000 times brighter than 

 the average solar star, and in that case the star might be seen at 

 V 1,000,000= 1,000 times that distance, or 3,375,000 light-years, 

 with an instrument having a space penetrating power no greater than 

 that employed by Herschel, provided that no light is extinguished in 

 its passage through space. 



(b) If the telescope be more powerful than Herschel's 20-foot 

 reflector, the light gathered will be increased in the ratio of .i'-/(i8)", 

 where .x- = diameter of mirror; and for the 60-inch reflector at 

 Pasadena, .r=^6o, over nine times as much light could be gathered, 

 or stars seen over three times as far away. Thus if the stars have 

 only about 10,000 times the luminosity of the sun, they could still 

 be seen with the Pasadena reflector at a distance of over a million 

 light-years. For 3,375 l.-y. X 3 X 100=1,012,500 light-years. 



(c) The sensitiveness and accumulative effects of the photo- 

 graphic plate, will enable us to extend our sounding line still further 

 out into space by some three magnitudes, or four times the distance ; 

 and thus with a modern 60-incli reflector we could photograph stars 

 at a distance of about four million light-years, if they have 10,000 

 times the standard solar luminosity, and no light is lost in space. 

 How much light is really lost in space will be considered later, but 

 it may be stated here that it probably is decidedly less than was con- 

 cluded by Struve. 



§ 3. Independent Calculation of the Distance of the 

 Remotest Stars of the Helium Type. 



From the data given in Lick Observatory Bulletin No. 195, we 

 find that 225 helium stars employed by Campbell in his line of 

 sight work have an average visual magnitude of 4.14. Of the four 

 variables given in this Bulletin, we have used the maximum bright- 

 ness in three cases, because they are of the algol type. In the case 



