396 



THE POPULAR EDUCATOR. 



LESSONS IN" ASTRONOMY. XVI. 



THE SPECTROSCOPE EXAMINATION OF BED FLAMES BY MEANS 



OF IT LUNAR ECLIPSES OCCULTATIONS ASTRONOMICAL 



INSTRUMENTS TELESCOPES TRANSIT INSTRUMENT. 



N the last few years our acquaintance with the physical con- 

 stitution of the Sun and stars has been much increased by the 

 Invention of the spectroscope. 

 Sir Isaac Newton made, many 

 years ago, the discovery that a 

 ray of white light could, by 

 means of a prism, be decomposed 

 so as to produce a spectrum ; 

 that it consisted, in fact, of 

 rays of the seven prismatic co- 

 lours all blended together. On 

 examining the solar spectrum 

 thus produced, Dr. Wollaston 

 found that, under certain con- 

 ditions, dark lines were to be seen crossing 

 it in different parts. These lines were fur- 

 ther found always to occupy the same place, 

 and the principal ones were accurately 

 mapped out by Fraunhofer, after whom they 

 are named. 



For some time, however, nothing arose 

 from this discovery, nor was any satisfactory 

 theory started to account for the lines. Seve- 

 ral philosophers were, however, engaged in 

 investigating the subject, among whom Pro- 

 fessor Kirchhoff should be specially men- 

 tioned. The rays from sources of light 

 were made to pass through prisms, and the 

 spectra thus obtained were examined to as- 

 certain whether any lines were present in 

 them. At length it was discovered that when 

 bodies are reduced to the state of a gas, and 

 burn, the spectra formed are not continuous, 

 but consist only of a series of bright lines, 

 and that these lines vary in colour and posi- 

 tion according to the substance burnt. If, 

 however, the rays of light from a luminous 

 body pass through any gas, then in the 

 place of the bright lines produced by that 

 gas, we have a corresponding series of dark 

 lines. 



Now the spectroscope is an instrument for 

 thus examining the rays of light from various 

 sources. A narrow ray of light enters the in- 

 strument through a telescope ; it then passes 

 through a series of prisms, by which it be- 

 comes opened out into a broad spectrum ; 

 end a second telescope, fitted with a micro- 

 meter, is provided for examining this spec- 

 trum, and ascertaining the position of the 

 lines. Pieces of platinum wire dipped in vari- 

 ous salts are then placed in the flame of a 

 spirit-lamp, and the lines produced by each 

 are carefully examined, and their places noted 

 down ; and in this way the lines charac- 

 teristic of most of the elements are ascer- 

 tained. The lines seen in the spectra of the 

 Sun and stars are then compared with these, 

 and thus many of the elements which exist 

 in those bodies have been discovered. The 

 Sun, for instance, is found to contain the 

 following elements : Sodium, iron, magne- 

 sium, hydrogen, barium, copper, zinc, calcium, 

 and several others. 



In the total eclipse of 1868, referred to in 

 our last lesson, the spectroscope was directed to the red protu- 

 berances with a view to ascertain something of their nature, 

 and bands indicative of hydrogen, sodium, and magnesium were 

 clearly traced in the spectrum. 



It seems clear, then, that they belong to the solar atmosphere, 

 and consist of gas in a state of ignition. Some of them, indeed, 

 appear to be clouds or mountains of flame, and they are fre- 

 quently in a state of violent agitation. 



These protuberances have now been seen at other periods 

 during an eclipse, however, is the most favourable time for 

 observing them. 



In history we find many notices of total eclipses, some of 

 which created great alarm. Xenophon, in the "Anabasis," 

 relates that a city called Larissa was besieged by the Persians, 

 but that they were unable to take it until a cloud covered the sun, 

 and caused it entirely to disap- 

 pear, when the inhabitants with- 

 drew in alarm. In one or two 

 instances battles were inter- 

 rupted by eclipses ; and at other 

 times they were considered to 

 foretell the death of monarchs, 

 or other calamities. Science 

 now, however, banishes all such 

 ideas, showing us that these 

 strange phenomena are but a 

 part of the regular mechanism 

 of the heavens, and teaching us the laws 

 which govern them, and by which we can 

 foretell the very moment at which they will 

 happen. 



We must now turn our attention from 

 eclipses of the Sun to those of the Moon. 

 These are greatly inferior in interest to solar 

 ones. There are, however, many points in 

 connection with them which repay observa- 

 tion. 



The main feature of difference between solar 

 and lunar eclipses consists in the fact that in 

 the former the light of the Sun is not actu- 

 ally diminished ; the dark body of the Moon 

 merely keeps off its rays from a portion of 

 the Earth's surface. The Moon, on the other 

 hand, shines only by reflected light, and an 

 eclipse of it is caused, not by any body inter- 

 vening between it and the Earth, but by the 

 light of the Sun being cut off from it, so that 

 the Moon for the time really ceases to shine. 

 By reason of this an eclipse of the Moon 

 is visible over one-half of the surface of the 

 grlobe that is, in all places where the Moon 

 itself is seen ; while a solar eclipse is, as we 

 have seen, confined to a comparatively limited 

 area. Hence, though solar eclipses are more 

 frequent than lunar ones, in the proportion of 

 about three to two, the solar eclipses, seen 

 at any given place of observation, are not 

 one-half as many as the lunar eclipses visible 

 at the same place during the same period. 



The theory of lunar eclipses will easily be 

 understood by reference to the annexed 

 sketch (Fig. 37) ; but it must, of course, be 

 remembered that here, as in our diagram of 

 a solar eclipse, the proportions are altogether 

 exaggerated to render the drawing more dis- 

 tinct. 



s represents the Sun, and E the Earth, the 

 shadow of which is a long cone reaching into 

 space a long way beyond the orbit of the 

 Moon. This dark shadow is the umbra, and 

 it gradually shades off into the penumbra, 

 which is bounded by the lines B D, A F, and 

 tapers towards the Earth, instead of away 

 from it. 



M represents the Moon revolving around the 

 Eirth, and in its journey it sometimes passes 

 through the dark cone, and thus becomes, for 

 the time, almost invisible. The commencement 

 of the eclipse is marked by a faint shade beginning to creep over 

 the east side of the Moon's disc. The moment of this occurring 

 is that mentioned in the tables as the first contact with the 

 penumbra. As the Moon travels onward it enters the umbra, 

 and the east side of its disc then becomes almost invisible 

 When fully immersed in the ucibra the Moon may usually be 

 feebly seen, and appears of a ruddy hue. The reason of this 

 was for some time a mystery, but it is now known, to arise from 



