370 



SCIENCE. 



[N. S. Vol. XVI. No. 401. 



termine the relation of the stars to each 

 other. The catalogue to be published is 

 likely to contain about 2,000,000 stars down 

 to the 11th magnitixde. When done, we 

 shall have the most valuable and extensive 

 star catalogue ever constructed. 



In addition to these plates the observa- 

 tories doing this work will also take plates 

 with exposures lasting thirty to fifty min- 

 utes (depending on the atmospheric condi- 

 tions). These i^lates Avill probably show 

 some 20,000,000 stars. 



To measure their positions and to reduce 

 the measurements would require much time 

 and money — more than the astronomers 

 and their patrons can. afford to give. It 

 has been decided, however, to enlarge these 

 plates by projjer lenses and to make a helio- 

 gravure of the enlargement. The liberal 

 French government has been the first to 

 publish a large number of these charts, 

 which show stars down to the fourteenth 

 magnitude and are invaluable for studying 

 at leisure a given part of the sky. Bach 

 plate covers about four square degrees. 



In our own country Professor E. C. Pick- 

 ering, of Harvard College Observatory, has 

 employed the Bruce telescope and other in- 

 struments to make photographs of the 

 heavens. Pickering by his system is able 

 to take a larger area on each plate and 

 finish his survey in a shorter time. He has 

 thus been able to collect a magnificent li- 

 brary of plates which have proved most 

 valuable in the past and are likely to prove 

 more precious in the future. Professor 

 Barnard and others have given considerable 

 time to using instruments which show large 

 areas of the heavens with exposures of sev- 

 eral hours. The wonder-exciting result is 

 obtained showing that the number of stars 

 goes on increasing. "V\Tien will it end? 

 What does it mean ? The astronomer bows 

 his head in awe-full ignorance! 



To-day we are alt amazed by the promises 

 of wireless telegraphy. IMessages across the 



ocean seem likely to be coming, soon from 

 every direction without going through 

 cables. 



Wireless telegraphic communication with 

 the sun, planets and stars the astronomer 

 has had for some time past. The messages 

 are received by a specially devised appara- 

 tus called a photospectroscope, and the 

 cipher dispatches are styled spectra. These 

 spectra are photographed on glass and are 

 measured, reduced and interpreted by the 

 expert. In the use of this instrument our 

 own country has done much, and the names 

 of Young, Pickering, Langiey, Keeler, 

 Campbell, Hale and others stand high in 

 the list of astrophysicists. 



What are the stars made of ? What ma- 

 terials are in the sun and in comets? in 

 nebulffi? The light from these bodies 

 speeds onward with a velocity of over 180,- 

 000 miles a second and takes more than 

 four years to come from the nearest star. 

 Even to come from the sun requires about 

 500 seconds of time. These light vibrations 

 enter the telescope and pass into the spec- 

 troscope, and proper apparatus enables us 

 to obtain a message which tells us what are 

 the gases in the sun, stars, comets and neb- 

 ulas. 



Moreover, this resultful instrument gives 

 us the power to determine motion and its 

 rate to and from the observer. The stars 

 are so distant that a line 93 millions of 

 miles in length would look to the inhabitant 

 of the nearest star as a line about two- 

 tenths of an inch long woiild appear to you 

 when placed a mile away ! . 



Motion to or from us of an object so far 

 away has hitherto been impossible to meas- 

 ure. The spectroscope solves the problem. 

 If a star is moving toward us then there is 

 a displacement of lines in the stellar spec- 

 trum toward the blue end, and if it is going 

 from us the displacement is to the red end. 

 By proper comparison-measurements the 

 rate of motion can be worked out. This 



