1862.] 63 



wires was also ascertained by observations of the sun made on each 

 side of the meridian ; this was necessary, because, in consequence 

 of the weather, the pole of the heliograph could be only approxi- 

 mately adjusted in position. 



Upwards of fifty plates were placed in the heliograph between 

 ll h 28 m A.M. and 4 h 16 m P.M. on July 18th ; some before the com- 

 mencement of the eclipse, and some after. During totality two pho- 

 tographs were obtained. One picture was produced on a plate which 

 was exposed from the exact commencement of totality during the 

 minute succeeding this epoch ; the second picture was exposed from 

 about a minute previous to the reappearance of the sun until not 

 more than a second before he became visible. In these pictures the 

 several prominences are depicted with great clearness ; and when one 

 negative is superposed on the other, corresponding parts exactly 

 coincide. During the taking of the second photograph, an excusable 

 curiosity on the part of two of the assistants disturbed the telescope 

 twice, so that the prominences have depicted themselves three times ; 

 but there was no difficulty in stopping out the images not belonging 

 to either of the three phases thus recorded. The author has more- 

 over turned this accident to account, and estimated the relative 

 brightness of the prominences in comparison with the sun's photo- 

 sphere ; and he considers that they are at least 600 times less bril- 

 liant than it. This conclusion has been drawn from the minimum 

 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- 



