1868.] 



Solar Eclipse 0/I868. 



115 



pedition deputed by the illustrious body I have now the honour of ad- 

 dressing — a responsibility more engrossing, it may be, but not lessened, by 

 the specific but novel character of the proposed object of the expedition. 

 I proceed now to describe how far that object has been attained ; and here 

 I feel that I cannot well indulge in too great a minuteness of detail. 



32. Relative positions of Pointer, Slit, and Sun's Limb. 

 The spectroscope may be inserted, and employed with its slit in any di- 

 rection perpendicular to the optical axis of the telescope. It is therefore 

 competent to the observer to place the slit perpendicular or tangential to 

 the sun's circumference at any point ; and there can be no doubt that, were 

 the observations conducted at leisure, it would be desirable to examine the 

 whole circumference in both positions ; but the operation of turning the 

 spectroscope is not so very simple a one but that the advantages and disad- 

 vantages of any such proceeding require to be well considered where time 

 is of the first importance. I decided on employing the slit in one direction 

 only, that which corresponded with the diurnal motion. It so happened 

 that this corresponded nearly with the direction of the relative motion of 

 the sun and moon, so that the widest part of the crescent could be made to 

 fall nearly perpendicularly across the slit. The needle (in the finder) and 

 its point accurately represented the direction and centre respectively of the 

 slit ; therefore, when the needle-point touched the sun's limb at the centre 

 of the crescent, a solar spectrum of definite width appeared in the field a 

 of which one edge (the right-hand) continued stationary, while the other 

 (the left) advanced slowly but perceptibly towards it, the solar spectrum 

 decreasing visibly in width. 



33. Last view of Solar Spectrum. 



About a minute's breadth remained. A few seconds more and it would 

 vanish suddenly. "Whatever spectrum the corona could show must then be 

 revealed, unless indeed a "prominence " or " sierra" should happen to be 

 situated at that precise spot, in which case the doable spectrum should be 

 presented. The nervous tension at the moment may be conceived : what 

 would be seen ? what call for action would be made ? and for what action ? 

 or, if nothing were seen, what would have to be done ? I cannot say that I 

 was prepared, at that moment, either with these questions, or with ready 

 answers to them ; but that was the sensation. With regard to the last, I 

 suppose I should have instinctively widened the slit ; and had that failed, 

 should then have gone to the finder to look for a prominence. As it was, 

 the spectrum faded out as I looked, while it had still appreciable width, and 

 I knew a cloud had intervened. 



Totality commences unseen. — A few seconds more and the spectrum of 

 diffuse light vanished also, and told me the eclipse was total, but behind a 

 cloud. 



34. On the watch for a glimpse. 



I went to the finder, removed the dark glass, and waited ; how long, I 



