442 Mr. Warren De la Rue — Comparison of De la Rue's [Recess^ 



forms which I have considered in my memoir on the Calculus of Functions 

 published in the Philosophical Transactions for 1862, in which the general 

 solution of the equations 



where ^ is the unknown function, has been obtained. 



Communications received since the end of the session. 



I. " Comparison of Mr. De la Rue^s and Padre Secchi's Eclipse 

 Photographs." By Warren De la Rue^ F.R.S. Received 

 August 8, 1864. 



I have stated, in the Bakerian Lecture read at the Royal Society on 

 April 10, 1862, that the boomerang (prominence E)* was not depicted on 

 Senor Aguilar's photographs. This is true of the prints which came into 

 my hands in England. A visit to Rome in November 1862, however, 

 afforded an opportunity for the examination of the first prints which had 

 been taken in Spain on the day of the eclipse, previous to those printed off 

 for general distribution by Senor Aguilar. I was agreeably surprised to 

 find that the photograph of the first phase of totality showed not only this 

 prominence very distinctly, but also other details, presently to be described, 

 which were quite invisible in Senor Aguilar's copies. I had in fact experi- 

 enced some difficulty in comparing measurements of my photographs with 

 those of Senor Aguilar's, on account of the indistinctness (wooUiness) of the 

 latter, which I have attributed to Padre Secchi's telescope not having fol- 

 lowed the sun's motion perfectly. A careful examination of the prints in 

 Padre Secchi's possession has, however, convinced me that this was not the 

 case during the period of exposure of the first negative ; for I have been able 

 to identify with a magnifier many minute forms which could only have been 

 depicted by the most perfect following of the sun's apparent motion. For 

 instance, my statement that the prominence H (the fallen tree) was not seen 

 from having been mixed up with the prominence G, is not applicable to 

 Padre Secchi's copy of the first phase of totality, for in it every detail of 

 the fallen tree can be made out. 



On expressing to Professor ISecchi my surprise at the great discordance 

 between the copy of the first phase of totality sent to me by Senor Aguilar 

 and that of the same phase in his possession, I was informed that after a 

 few positive prints had been taken from the then unvarnished negative, it 

 was strengthened by the usual photographic process with nitrate of silver. 

 This I look upon as an unfortunate mistake, as the images of the promi- 

 nences were increased and their details hidden, and the beauty of the 

 negative for ever lost. 



It occurred to Padre Secchi and myself that although there was no hope 



* See Index Map, Plate XV. Phil. Trans. Part I. 1862. 



