93 THE CHEMISTRY OF THE SUN. [CHAP. 



2. How these Details can be Spectroscopically Observed. 



Now it is evident that if we simply direct a spectroscope 

 to the sun, or reflect sun-light into it, it will receive light 

 coming indiscriminately from spots, faculse, and general surface, 

 and if there are any differences in the spectra of these regions, 

 in other words, if the chemistry of the various regions is different, 

 the differences will be lost, since we shall obtain a mixed spec- 

 trum partly due to the spots, partly to the faculae, and partly to 

 the general surface. 



But there is another way of observing the spectrum of the 

 sun. We can throw an image of the sun, or of any part of the 

 sun, on the slit of the spectroscope. This kind of work, though, 

 as we have seen, it was suggested by Forbes thirty years earlier, 

 and actually employed by Angstrom to investigate the -darken- 

 ing of the limb, was first applied to the spots and faculai 

 in 1866. 



If a spot or a facula be visible on the sun, we can throw 

 the image of the sun on the slit plate, and then bring it exactly 

 on the slit. We shall in this way get the spectrum of the sun- 

 spot, or facula, as distinguished from the spectrum of the other 

 portions of the sun. 



The manner in which this kind of work is carried on is easily 

 grasped. It simply consists in the use of a spectroscope of 

 large dispersion attached at the focal point of a telescope of 

 considerable power. 



Fig. 37 shows the eyepiece end of Mr. Newall's giant refractor, 

 with a spectroscope with a considerable number of prisms, lixed 

 to the telescope by means of an iron bar. The slit of the 

 spectroscope occupies the focus, so that when the instrument is 

 pointed towards the sun we see an image in the case of this 

 telescope something like four inches in diameter with the spots 

 and brighter portions wonderfully and beautifully clear ; and by 

 means of the different adjustments of the telescope we can 



