510 



On Spectroscopic Observations of the Sun. 



[June 17, 



Neither word even occurs in my report, except incidentally in the words 

 " a sodium-flame." The exceeding care which I observe in your own 

 writings in this respect will, I hope, make it unnecessary for me to apo- 

 logize for this protest. 



To return : I do not question the presence of an element giving a green 

 line ; the testimony of three observers must be taken as conclusive ; but I 

 am slow to believe that either E or b is connected with it. 



This afternoon I made a slight advance towards seeing the actual forms 

 with greater comfort. Taking a hint from your figure (p. 216 of the above 

 article), I introduced one of the compound prisms of the hand-spectro- 

 scopes into the eye-tube, thus increasing the dispersion by about one-fourth, 

 which enabled me to open the slit a little wider. Finding a suitable pro- 

 minence, I was able to examine it with an aperture of about 20" to 25". 

 As it was not much more than V in height, I could, by a slight pressure 



one way or the other, f 



view the whole. It is of ^ ^ ^ 



course wholly unimpor- C 

 tant what the actual form 

 was, but, for the sake of _ 



illustration, I attempt a j \ ^ 



drawing. It is not easy ~ 

 to convey the impression ~s 

 of a fleecy cloud such as 

 I saw. I looked at one or two others in the same way, and left off even- 

 tually quite satisfied that with a suitable battery the whole of any promi- 

 nence or eruption might be seen with comfort (either the red, or the 

 orange, or the blue, or any other principal image being examined at will) 

 by limiting the field of view, and with it the unnecessary diffuse light, to 

 the actual dimensions of the object. The portion of a cloud-shape which 

 is due to one element will thus be artificially separated from the form which 

 is due to another, and the regions or strata to which the various elements 

 are confined will become known with certainty*. 



It is unfortunately impossible for me to prosecute these researches any 

 further. I have neither the leisure nor the opportunity to devise and use 

 suitable instruments except at rare intervals, for which such discoveries 

 will not wait. Yours very truly, 



J. Herschel, Lt. R.E. 



* As an instance of this kind, I may point to Captain Haig's observations with the 

 hand-spectroscope. As this instrument has no slit, his " bands " mean the coloured re- 

 petitions of the line of sierra or low clouds fringing the moon's limb at the point, only 

 that with so low a power, and amid the confusion of images, he did not recognize (ap- 

 parently) the similarity of general outline of the differently coloured images. Hence 

 the term " bands," which has misled at least one reviewer into inferring a slit, and 

 thereby immensely overrating the scope of these instruments. 



J. H. 



