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this discovery was nearly two months in reaching Em-ope, and 

 meanwhile Mr. Lockyer had succeeded, perfectly independently 

 of Janssen, in viewing the spectrum of the prominences in the 

 same manner. The idea he had first conceived in 1866, but 

 want of instrumental power had hitherto prevented him from 

 successfully testing his views. The credit therefore of this most 

 important discovery is equally and most deservedly shared by 

 these two eminent men of science. 



Before proceeding to explain the principles which enable us to 

 study the prominences without the aid of an echpse, it may be 

 well to give a brief description of the essentials of the telespec- 

 troscope, the instrument employed in such researches. The rays 

 of light proceeding from the sun, forming a parallel pencil of 

 rays on account of its distance, are first received by the object- 

 glass of a telescope, which being a convex-lens, causes them to 

 converge and form an image in the focus of the glass. In this 

 focus is placed the sUt-plate, in which is the delicate sUt adjust- 

 able within certain limits, ordinarily by means of a screw acting 

 on a spring. The hne of Ught admitted by the sUt forms a 

 diverging pencil, the rays of which are caught by a second lens 

 and rendered parallel before passing through the prism or prisms 

 which break the light up into its constituent parts. A small 

 telescope is placed beyond the prisms, in the eye-piece of which 

 a magnified image of the spectral band may be viewed. We 

 ordinarily see the sun by two sets of rays, first by its direct rays, 

 and secondly by those which are reflected to the eye by our 

 atmosphere. "When ui an eclipse the dark body of the moon is 

 interposed between us and the sun, not only are the direct rays 

 cut off, but also in a less degree the reflected rays, so that there 

 is total darkness except for a lurid glow about the horizon. To 

 a certain extent the slit-plate of a spectroscope performs the duty 

 of the moon during an echpse, for by placing the image of the 

 sun a little below the slit, no dh-ect rays of sun-light are able to 

 enter through it. In this position we have the slit across the 

 regions of the solar atmosphere, and at first sight it would seem 

 that we ought to be able to view the chromospheric spectrum. 

 But we have not yet dealt with the reflected sun-light, which is 

 especially strong in the immediate neighbourhood of the limb, 

 so strong, that in a spectroscope of small dispersive power, the 

 spectrum of the solar atmosphere would be altogether masked 

 by the continuous spectrum of the nucleus, reflected from the 

 terrestrial atmosphere. It is precisely to the device Mr. Lockyer 

 suggested for overcoming this difficulty, that is due the credit of 

 the successful method he proposed for viewing the prominence 

 spectrum in ordinary dayhght. He argued thus. By increasing 

 the battery of prisms it wiU be possible to dilute the band of 

 colours reflected from the atmosphere until it becomes very faint. 



