94 REPORT — 1861. 



Report on the Progress of Celestial Photography since the Aberdeen 

 Meeting. By Warren De la Rce, F.R.S. 



At the Aberdeen Meeting I had the honour of communicating to this 

 Section a Report on the State of Celestial Photography in England, which 

 has since appeared in the Transactions of the Association. 



Since that period I have pursued my investigations in this branch of 

 astronomy, and have ascertained some facts which 1 believe will be of interest 

 to the Meeting. 



In the first place I beg to recall to the recollection of Members who may 

 have read my paper, and to re-state for the information of those who have not 

 done so, that it was intended at the period of the Aberdeen meeting that the 

 Kew photo-heliograph should be taken to Spain in order, if possible, to photo- 

 graph the luminous prominences, or, as they are usually called, the red 

 flames, seen on the occasion of a total solar eclipse. 



The words implying a doubt as to the success of the undertaking were 

 advisedly inserted, because very little information could be collected from the 

 accounts of those observers who had witnessed previous total eclipses, as to 

 the probable intensity of the light of the corona and red flames in comparison 

 with that of other luminous bodies. My impression was that I should fail in 

 depicting the prominences in the time available for doing so, because I had had 

 the Kew instrument tried upon the moon and had failed utterly in getting even 

 a trace of her image on the sensitive plate, and the corona and prominences 

 together were not supposed to give as much light as the moon. I therefore 

 pointed out the desirability of other astronomers making attempts to depict 

 the phenomena of totality by projecting the image of the prominences direct 

 on to the collodion-plate without enlarging it by a secondary magnifier, as is 

 done in the Kew instrument. 



It was fair to assume, with the great experience I had acquired in celestial 

 photography, that I should succeed with the Kew instrument if success were 

 attainable ; and I knew that far more reliable results would be obtained by 

 its means than by the other method, which I recommended simply because it 

 seemed to me to offer a greater chance of at least a partial success. 



Two theories existed, as is well known, to account for the red prominences. 

 The one, prominently supported by the Astronomer Royal, was that they 

 belonged to the sun ; the other, which is still supported even by an astrono- 

 mer who obtained photographs of them at the last eclipse, was that they are 

 produced by the diff"raction of the sun's light by the periphery of the moon. 



It will be seen, therefore, not only how essential it was to obtain photo- 

 graphic images of the prominences, but also how important it was to obtain 

 such perfect images of them that they could not be confounded with 

 purely diff'ractive phenomena if such existed, and that the images should be 

 on such a scale that the defects common to collodion could not be confounded 

 with them. "The pretty near" would have been far more readily accom- 

 plished ; but having the whole bearing of the subject fully impressed on 

 my mind, I preferred to make a bold venture, and either accomplish what I 

 aimed at or fail entirely. 



Fortunately 1 was successful, and to that success the steadiness of my 

 staff much contributed. We now know that the luminous prominences 

 which surround the sun (for they do belong to him) can be depicted in 

 from 20 to 60 seconds, on the scale of the sun's diameter equal ^ of the 

 object-glass employed. That is to say, an object-glass of 3 inches aperture 

 will give a picture of the prominences surrounding the moon 4 inches in 

 diameter. 



