330 REPORT— 1872. 



culties as to travelling expenses from the places of observation to Galle or 

 Bombay, by a liberal grant of ^100. 



In conclusion the Committee have much pleasure in laying before Section A 

 an interim report by Mr. Lockyer on the results of the expedition, to be fol- 

 lowed as soon as possible by the full report, which the Royal Astronomical 

 Society have undertaken to publish. 



An interim Report on the Results obtained hy the British- Association Eclipse 

 Expedition o/1871. By J. Norman Looktek, F.R.S. 



I. New Instruments. 

 These were as follows : — 



1. A train of five prisms to view the corona. 



2. A large prism of small angle placed before the object-glass of a telescope. 

 On these instruments I may remark that the Eoyal Astronomical Society, 



in the first instance, invited me to take charge of an Expedition to India 

 merely to conduct spectroscopic observations ; but although this request did 

 me infinite honour, I declined it, because the spectroscope alone, as it had 

 been used before, was, in my opinion, not competent to deal with all the 

 questions now under discussion. Thus some of the most eminent American 

 observers had come to the conclusion that the spectrum of hydrogen observed 

 in the last eclipse round the sun, to a height of 8', was a spectrum of hydrogen 

 " far above any possible hydrogen" at the sun. Hence it was in some way 

 reflected. Now with our ordinary spectroscopic methods it was extremely 

 difficult, and one might say impossible, to determine whether the light which 

 the spectroscope analyzed was reaUy reflected or not ; and that was the whole 

 question. 



It became necessary, therefore, in order to give any approach to hopeful- 

 ness, to proceed in a somewhat different way in the 1871 expedition, with 

 regard to the spectroscope, and, to guard against failure, to supplement such 

 observations with photographs. 



To understand the method adopted, let us suppose a train of prisms. Take 

 one prism out of the train, and consider what will happen if we illuminate a 

 slit with a monochromatic light and observe it through the prism. If we 

 render sodium vapour incandescent and illuminate the slit by means of it, we 

 get a bright yellow image of the slit, due to tlie vapour of the metallic sodium 

 only giving us yellow light. But why is it that we get a line ? Because we 

 employ a line slit. If, instead of a straight line, we have a crooked line for 

 the slit, then we see a crooked line through the prism. Going one step 

 further : suppose that instead of a line, whether straight or crooked, we have 

 a slit in the shape of a ring, we see a ring image through the prism. And 

 then comes this point : if, when we work in the laboratory, we examine these 

 various slits, illuminated by these various vapours, if we observe the corona 

 in the same way, we shall get a ring built up by each ray of light which the 

 corona gives to us, since we know, from the American observations, that 

 there were bright lines in the spectrum of the corona as observed by a line 

 slit ; in other words, the corona examined by means of a long train of prisms 

 should give us an image of itself painted by each ray which the corona is 

 competent to radiate towards us*. 



These were the considerations which led to the adoption of this new attempt 

 to investigate the nature of the corona now in question. It was, to use a 



* After I had thought of this arrangement, and had secui-ed an instrument to carry it out, 

 Prof. Young, in a communication to ' Nature,' suggested the same method of observation. 



