Mr. De la Rue on the Total Solar Eclipse of July 18, 1860. 73 



time required by the prominences to depict themselves, which can be 

 made out from the photograph in question. 



By means of a new micrometer contrived for that purpose by the 

 author, the several photographs have been measured and discussed. 

 The position-angles of the line joining the sun's centre and the 

 moon's centre, and the distances of these centres for the several 

 epochs of the photographs, have been calculated and compared with 

 the corresponding values calculated by Mr. Farley for the geogra- 

 phical position of the observatory. Other calculations have also been 

 made from the photographs and compared with certain elements of 

 the eclipse calculated by Mr. Carrington. The results show that the 

 photographic method of observing solar phenomena is capable of 

 great exactness. 



The nearest approach of the centres of the sun and moon, as ascer- 

 tained from the photographic measurements, was 1 1"*8, calculation 

 giving as a mean 12"*8. The relative diameter of the moon, that of 

 the sun being taken as unity, as derived from measurements of the 

 photographs, comes out 1*0511, which is precisely the theoretical 

 number ; on the other hand, they tend to show that the diameters at 

 present assumed for the sun and moon, taken conjointly, are about 

 4"*0 in excess of the truth. 



The paper is accompanied by an extensive series of calculations, 

 which it is not here necessary to describe. Those, however, relating 

 to the measurements of the positions of the luminous prominences 

 on the two totality-pictures have especial interest. These measure- 

 ments were made in two ways: 1st, the original negatives were 

 measured by the author's new micrometer; 2nd, enlarged positive 

 copies were taken on glass, and the contours of the prominences 

 traced and etched upon the glass, which was afterwards centered on 

 a dividing engine and divided, the divisions being subsequently 

 etched. Copper duplicates were then made of the glass plates, 

 which served to print off diagrams which accompany the paper. 



Without describing minutely the measurements, it will suffice here 

 to state that the results go to prove that the luminous prominences 

 must belong to the sun and not to the moon. For example, the 

 change in the angular position of the prominence at a right angle to 

 the moon's path, and designated A in the paper, has been calculated 

 to have been 5° 21' for the assumed geographical position of the 

 station ; by measurement of the two photographs it is 5° 32 r . The 

 motion of the moon in covering and uncovering a prominence nearly 

 in the line of her path was calculated to have been 92"' 1 ; by mea- 

 surement it was found to have been 93"*0. The accordance of these 

 numbers is so extremely close, that it would be difficult to obtain 

 more convincing proofs that the luminous prominences belong to the 

 sun. 



