But to return to the sun. In your small spectroscopes you see very few dark lines, but in larger and more perfect ones they can be counted by thousands, and can be compared with the bright lines of glowing gases burnt here on earth. In the spectrum of glowing iron vapour 460 lines are found to agree with dark lines in the sun-spectrum, and other gases have nearly as many. Still, though thousands of lines can now be explained, by matching them with the bright lines of known gases, the whole secret of sunlight is not yet solved, for the larger number of lines still remain a riddle to be read.
We see then that the spectroscope teaches us that the round light-giving disc or photosphere of the sun consists of a bright and intensely hot light shining behind a layer of cooler though still very hot vapours, which form a kind of shell of luminous clouds around it, and in this shell, or reversing layer—as it is often called, because it turns light to darkness—we have proved that iron, lead, copper, zinc, aluminum, magnesium, potassium, sodium, carbon, hydrogen, and many other substances common to our earth, exist in a state of vapour for a depth of perhaps 1000 miles.
You will easily understand that when the spectroscope had told so much, astronomers were eager to learn what it would reveal about the prominences or red jets seen during eclipses, and they got an answer in India during that same eclipse of August 1868 which is shown in our diagram (Fig. 47). Making use of the time during which the prominences were seen, they turned the telescope upon them with a spectroscope attached to it, and saw a number of bright lines start out, of which the chief were the three bright lines of hydrogen, showing that these curious appearances are really flames of glowing gas.
In the same year Professor Jannsen and Mr. Lockyer succeeded in seeing the bright lines of the prominences in full sunlight. This was done in a very simple way, when once it was discovered to be possible, and though my apparatus (Fig. 49) is very primitive compared with some now made, it will serve to explain the method.
The spectroscope attached to the telescope for the examination of the sun. (Lockyer.)
P, Pillar of Telescope. T, Telescope. S, Finder or small telescope for pointing the telescope in position. a, a, b, Supports fastening the spectroscope to the telescope. d, Collimator or tube carrying the slit at the end nearest the telescope, and a lens at the other end to render the rays parallel. c, Plate on which the prisms are fixed. e, Small telescope through which the observer examines the spectrum after the ray has been dispersed in the prisms. h, Micrometer for measuring the relative distance of the lines.
When an astronomer wishes to examine the spectrum of any special part of the sun, he takes off the eye-piece of his telescope and screws the spectroscope upon the draw-tube. The spectroscope is made exactly like the large one for ordinary work. The tube d (Fig. 49) carries the slit at the end nearest the telescope, and this slit must be so placed as to stand precisely at the principal focus of the lens where the sun's image is formed (see i, i, p. 44). This comes to exactly the same thing as if we could put the slit close against the face of the sun, so as to show only the small strip which it covers, and by moving it to one part or another of the image we can see any point that we wish and no other. The light then passes through the tube d into the round of prisms standing on the tray c, and the observer looking through the small telescope e sees the spectrum as it emerges from the last prism. In this way astronomers can examine the spectrum of a spot, or part of a spot, or of a bright streak, or any other mark on the sun's face.
Now in looking at the prominences we have seen that the difficulty is caused by the sunlight, between us and them, overpowering the bright lines of the gas, nor could we overcome this if it were not for a difference which exists between the two kinds of light. The more you disperse or spread out the continuous sun-spectrum the fainter it becomes, but in spreading out the bright lines of the gas you only send them farther and farther apart; they themselves remain almost as bright as ever. So, when the telescope forms an image of the red flame in front of the slit, though the glowing gas and the sunlight both send rays into the spectroscope, you have only to use enough prisms and arrange them in such a way that the sunlight is dispersed into a very long faint spectrum, and then the bright lines of the flames will stand out bright and clear. Of course only a small part of the long spectrum can be seen at once, and the lines must be studied separately. On the other hand, if you want to compare the strong light of the sun with the bright lines of the prominences, you place the slit just at the edge of the sun's image in the telescope, so that half the slit is on the sun's face and half on the prominence. The prisms then disperse the sunlight between you and the prominences, while they only lessen the strong light of the sun itself, which still shows clearly. In this way the two spectra are seen side by side and the dark and bright lines can be compared accurately together (see Fig. 50).