62 [April 10, 



moonlit night, although at the time the light appeared to the author 

 as less bright than what he remembered of bright moonlight. By 

 subsequent trials he was led to conclude that the light during a 

 total eclipse most resembles that degree of illumination which exists 

 in a clear sky soon after sunset, when, after having made out a first- 

 magnitude star, other stars of less brilliancy can be discerned one 

 after another by an attentive gazer. Jupiter and Venus were the 

 only objects the author had time to identify, but some neighbouring 

 observers saw also Castor. 



The most important part of the paper treats of the photographic 

 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, wben it struck 

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

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



