10 SEE— DETERMINATION OF THE [Januarys. 



tude of 4.14. This would almost have doubled the calculated depths 

 of the Milky Way throughout the foregoing discussion, and given 

 us over two million light-years, exceeding the profundity originally 

 concluded by Ilerschel in 1802. In the Phil. Trans, for 1800, pp. 

 83-4, Herschel finds by a different process that a cluster of 5,000 

 stars visible in his 40- foot telescope is distant 11,765,475,948,678,- 

 678,679 miles, " a number which exceeds the distance of the nearest 

 fixed star at least three hundred thousand times." With modern 

 data this proves to be 460,355 times the distance of Alpha Centauri, 

 or 2,001,120 light-years. 



§ 10. Summary of the Chief Results of the Determination 

 OF THE Depth of the Milky Way. 



From the several independent and mutually confirmatory argu- 

 ments here adduced it follows that the depth of the Milky Way 

 decidedly exceeds a million light-years, and substantially accords 

 with the profundity concluded by the illustrious Herschel one hun- 

 dred and ten years ago. 



1. Herschel concluded that with his forty-foot reflector he per- 

 ceived stars whose light had occupied two million years in reaching 

 the earth; and he justly remarked that he had seen further into 

 space than any human being before him. The visual power or light 

 grasp of Herschel's telescope is somewhat surpassed by modern 

 instruments; and much additional power is given to the modern 

 instrument by the use of photography. 



2. But if, on the one hand, the modern instruments surpass 

 Herschel's in power, there is on the other some increased need for 

 this in that we now attempt to take account of the extinction of light 

 by cosmical dust in space. Neglecting this loss of light, Herschel 

 may have slightly overestimated the distances to which his telescope 

 could penetrate, but the error was scarcely of sensible importance. 



3. With our greatest modern instruments and the use of pho- 

 tography it is certain that we can observe stars- at a distance of over 



*In Astron Nachr., No. 4.536, Nov. 13, 1911, Professor F. W. Very con- 

 cludes that the White Nebula may be galaxies at a distance of a million light- 



