62 THE INDIAN ECLIPSE, 1898. 



commonly known as "1474K," less bright than the greenish 

 blue line near it due to hydrogen, and known as " H/3," whereas 

 Pickering, observing with an integrating spectroscope, found 

 "1464K" distinctly the brighter of the two. This showed 

 that though "1474K" was intrinsically much fainter than 

 " H/3," yet it existed over a much wider area than the hydrogen 

 line. Less bright at one particular point, it gave a greater 

 total amount of light on the whole, as it extended over a wider 

 region. In short, "H/3" was a line of the prominences, but 

 " 1474 K " was a line of the corona. 



A third mode of using a spectroscope in an eclipse is dictated 

 by the fact that as the black disc of the moon cuts out the light 

 of the sun, the corona and prominences form as it were a rough 

 ring of light, and a slit of a ring form is for many purposes just 

 as useful in a spectroscope as the ordinary straight slit. If, 

 then, when using the analysing spectroscope, i.e. the spectro- 

 scope in connection with a large telescope, we dispense with the 

 slit, the eclipse itself will act as its own slit, and we shall have 

 the spectrum not of a narrow strip alone, as we should if we 

 used the slit, but of the entire phenomenon, every point of 

 which would give its own special and peculiar spectrum. This 

 method is usually briefly described as that of a " slit less 

 spectroscope." 



This method leads on to the fourth method. In this third 

 method an image of the eclipse is formed at the point where the 

 slit would have been, and the rays from that image are rendered 

 parallel by a collimating lens before they fall on the prism. 

 But we have already the actual eclipse itself up in the sky, and 

 the rays proceeding from it are already parallel. We may 

 discard, then, both the large telescope to form the image, and 

 the collimator, and use only the prism and the camera. This 

 brings our spectroscope down to its simplest form, and it is 

 generally described as a "prismatic camera," or sometimes as 

 the method of the "object-glass" or "objective prism." 



One great advantage of this form is that it embraces the 

 whole of the phenomenon in its grasp. Another is that it is 

 very economical of light. A third is that its simplicity renders 

 it possible to use a far larger prism and camera than could be 

 done if these were but parts of a spectroscope attached to a 

 larger telescope. Then it is the analysing spectroscope par 

 excellence, for every part of the eclipse gives its own peculiar and 

 separate spectrum ; each separate coloured ray of light paints 

 its own picture of the eclipse. 



These four kinds of spectroscope were all in use by Mr. Evershed 

 during the late eclipse. What may be described as a fifth form, 

 a "prismatic opera glass," was used by Mr. Maunder. This 

 consisted of a binocular, to one of the eyepieces of which a small 

 direct-vision prism was attached. This enabled the observer to 



