406 



THE GUIDE TO NATURE 



this sun and at intervals of two days and 

 eight hours partially hides it from our 

 view. The maximum darkening will oc- 

 cur this month on the nth and on the 

 1 8th at midnight and on the 25th at 

 about eleven hours and forty minutes 

 P. M. 



In spite of its peculiar color, the star 



Figure 4. Spectrum of a new star and of the sun. 



at B is known to be in very much the 

 same condition as the bluish Vega and 

 Spica and the great Sirius ; it is a star of 

 the so-called Sirian type in which the 

 lines of hydrogen are extraordinarily 

 conspicuous in the spectrum. X early 

 half of all the stars belong to this great 

 class, the supposedly older and more con- 

 densed stars which resemble our sun, 

 are but little less numerous, while the 

 conspicuously different sun — those of a 

 deep red color, the carbon stars, the very 

 old ones, etc. — comprise altogether 

 scarcely more than one-eighth of the to- 

 tal number to be seen in the sky. 



The star at B is also known to be ap- 

 proaching us with a speed of six miles a 

 second. 



How We Find What the Stars Are 

 Made of. 



Over a few years ago it would have 

 been thought quite impossible that men 

 could ever analyze the far distant stars 

 and nebulas and many other heavenly 

 bodies ; indeed, one of the greatest of the 

 philosophers referred to this as one of 

 the things which from its very nature 

 would remain forever unknown to us. 

 Yet the explanation of how we have 

 achieved this most remarkable know- 

 ledge is a very simple one. It has all 

 come through the discovery of the spec- 

 troscope, an instrument of so much im- 

 portance that it has given birth to an en- 

 tire new province of astronomy. It has 

 divided the whole science into two parts 



the older, or fundamental astronomy, 



and the so-called astro-physics. Many 

 great observatories are wholly devoted to 



either the first or to the second of these 

 lines of investigation. 



It is known to everyone that if light 

 is passed through a prism, as in Figure 

 2, it will be bent out of its course, so that 

 were a screen placed at MN the beam of 

 light would not fall on this screen at H, 

 but at the higher position RV. And at 

 RV we would see the color strip, or 

 spectrum of colors, merging from red at 

 its upper end to violet below. 



If the light, S, came from a brightly 

 shining, vaporized body and the slit K 

 was made very narrow, then the color 

 strip RY would be seen to be crossed by 

 many bright lines, and the position of 

 these lines would depend entirely on the 

 nature of the bright substance which 

 was emitting the light. Thus were the 

 bright substance, iron, so intensely heat- 

 ed that it had become a gas, we would 

 see thousands of lines crossing the color 

 strip, each one caused by iron-light, and 

 each one in a perfectly definite place in 

 the strip; no other substance could pro- 

 duce lines in the same positions as the 

 iron lines. 



The usual arrangement is as shown 

 in Figure 3. All light gathered by the 

 large lens of the telescope passes out of 

 the smaller end. Ki, and falls on the slit 

 which is at K2. The portion of the light 

 which passes through the very narrow 

 slit then enters the train of prisms, P, the 

 spectrum finally falling on the photo- 

 graphic plate at F. 



The upper color strip of Figure 4 is 

 from a bright star in Auriga, while the 

 lower is a part of the spectrum of our 

 sun. A mere glance shows that these 

 two bodies must be of a very different 

 constitution. In the latter we find the 

 lines of iron, carbon, silicon, hydrogen, 

 etc., in fact nearly all of the elements of 

 which our earth is composed. In this 

 way a new element, helium, was discov- 

 ered on the sun twenty-seven years be- 

 fore it was actually found on the earth. 



It is one of the most striking revela- 

 tions of spectroscopy that the infinite va- 

 riety of the heavenly bodies is composed 

 of the same limited number of elements 

 as make up our own earth. A uniform- 

 ity of substance thus extends throughout 

 our whole visible universe. 



I am like a feather floating in the at- 

 mosphere. On every side is depth un- 

 fathomable. — Thoreau. 



