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THE NATIONAL GEOGRAPHIC MAGAZINE 



the interval required for the revolving 

 mirror to move the distance between the 

 spot of my departure and that of my 

 return. They found, by noting the di- 

 rection I was hurled after returning, 

 that the mirror had turned 2^ degrees 

 between my going and coming, which, 

 at 250 revolutions per second, amounted 

 to 1 /40000th of a second. I had trav- 

 eled 4 l / 2 miles in that time. So they 

 knew that my velocity is 186,330 miles 

 per second — seven times around the 

 world before you can say 'Jack Robin- 

 son' ! Thus was Roemer's deduction 

 conclusively sustained. 



"Then other men invented a wonderful 

 instrument called the spectroscope which 

 forces me to write my life story on a 

 photographic plate (see page 162). By 

 this means they can tell whether I origi- 

 nated in an incandescent gas or from a 

 solid body ; whether or not I came 

 through a cool gas in leaving the star that 

 started me ; and, if, so, whether that gas 

 was under pressure or free. 



"Now every message I bring, whether 

 from the nearest planet, the farthest 

 star, or the remotest nebula, can be de- 

 coded and read. 



"In the words of Abbot, the message 

 may be faint and hard to read, but it tells 

 of the materials of which the stars are 

 made, their temperature, their velocity, 

 their brightness, their distance, etc." 



A WIRELESS WAVE WITNESS 



The last witness to the credibility of 

 the astronomer is the electromagnetic 

 wave. It deposes as follows : 



"Yes, I take my hat off to these astron- 

 omers. After that canny Roemer proved 

 that light is not instantaneous, another 

 eminent scientist undertook to find out 

 what it really consists of. By purely 

 mathematical processes, this Mr. Clerk- 

 Maxwell came to the conclusion that light 

 is a matter of waves, some of them inap- 

 preciably short and others tremendously 

 long; many too short to be seen and some 

 too long. 



"1 knew he was getting close to my 

 secret, for T am a long wave, sometimes 

 many miles long, whereas the X-rays are 

 often less than the billionth of an inch in 

 length. Then came another man. Hertz 

 by name. lie placed a great sheet of 



metal against the wall of a room and 

 sent me toward it. I was reflected like 

 sound by a sounding-board. There were 

 two points in the room where the spark 

 would not jump the gap. They were 

 half a wave-length distant from one an- 

 other. He was thus not only able to de- 

 tect me, but to measure my length and 

 my velocity. 



"Then Branley found how to make an 

 extremely sensitive detector which would 

 catch me. Sir Oliver Lodge developed 

 this into a coherer and employed it in 

 signaling. Wireless telegraphy followed 

 apace, and every boy who has a wireless 

 set uses me because these astronomers, 

 mathematicians, and physicists calculated, 

 detected, and harnessed me." 



Thus endeth the testimony, which could 

 be added to, corroborated, and reinforced 

 a thousandfold. 



A PENETRATING EYE 



A visit to an astronomical observatory 

 and a study there of two or three of the 

 instruments with which the astronomer 

 works gives some clue to the secret of 

 the vastness of his power, as compared 

 with the layman's, in penetrating the 

 mysteries of space. 



Of course, the first thing that claims 

 our attention is the big equatorial tele- 

 scope, which multiplies the power of the 

 astronomer's eye as much, perhaps, as a 

 locomotive throttle multiplies the power 

 of an engineer's arm. It is a far cry 

 from the lens fashioned from a block of 

 ice, with which Metius concentrated the 

 rays of the sun and set fire to a piece of 

 wood, to the great 100-inch reflecting 

 mirror of the new Mount Wilson tele- 

 scope (see pages 164 and 165). 



The pupil of the human eye is about 

 one-fifth of an inch in diameter. It 

 brings to a focus on the retina only so 

 many rays of light as fall within such 

 an area. If it were one inch in diameter 

 and could bring to a focus all the rays 

 entering it, our vision would be twenty- 

 five times as strong; if six inches, and the 

 rays entering could be centered on the 

 retina, we could see an object nine hun- 

 dred times as faint as those visible with 

 the unaided eye. 



We cannot regulate the size of the 

 pupils of our eyes at will, but we can 



