72 Royal Society : — 



observations. The several preparations are minutely described, and 

 drawings, showing the general arrangements of the observatory, are 

 given. In the focus of the secondary magnifier of the Kew helio- 

 graph, two position-wires, crossing at right angles, are fixed at 

 approximately an angle of 45° to a parallel of declination. The 

 object-glass has an aperture of 3*4 inches and a focal length of 

 50 inches : the primary focal image of the sun at his mean distance 

 is 0*47 inch ; but before it is allowed to fall on the sensitive plate, it 

 is enlarged to about 3'8 inches by means of an ordinary Huyghenian 

 eyepiece. The object-glass is so constructed as to ensure the coin- 

 cidence of the chemical and visual foci ; this coincidence is, however, 

 disturbed in a slight degree by the Huyghenian magnifier, which 

 renders a slight adjustment necessary. For ordinary sun-pictures, 

 and those of the several phases of the eclipse except the totality, the 

 aperture was reduced to 2 inches, — a peculiar instantaneous appara- 

 tus being employed to regulate the exposure of the sensitive plate. 



The driving-clock of the heliograph was, for convenience, kept 

 going during the taking of the partial phases of the eclipse ; but it 

 was not really necessary to keep it in motion, because the time of 

 exposure certainly did not exceed the -^th of a second. 



The position-wires, by stopping off the sun's light, are depicted in 

 the negatives as white lines crossing the solar disk. It was essential, 

 in order to turn these several pictures to account, to note exactly the 

 time of their being taken, which was done by Mr. Beckley; the click- 

 ing noise made by the instantaneous apparatus, when it struck 

 against a stop after releasement, indicating the epoch, which was 

 noted to the nearest half-second. The exact position of the cross 

 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 



