the Solar Eclipse, July 18, 1860. 193 



photographic surfaces were usually employed, for the chemical 

 and visual foci had previously been ascertained to be very nearly 

 coincident. The aperture of the object-glass was now stopped 

 down until, with a diaphragm of *25 inch, the exposure to the 

 powerful action of direct sunlight was rendered manageable ; and 

 in order to secure an easy and sufficiently rapid means of opening 

 and closing this small aperture, a card of about 6 inches square 

 was provided, having cut out from its centre a narrow slit of 

 about an eighth of an inch in diameter and nearly 2 inches in 

 length. This quickly moved by the hand in front of the dia- 

 phragm of the telescope lens, limited the period of exposure to 

 a small fraction of a second, and besides made it possible to 

 regulate the interval of time at the taking of each picture accord- 

 ing to circumstances at the moment, which were occasionally 

 varying as light fleecy clouds passed over the face of the sun. 

 Mr. Crookes, to whom I am indebted for taking charge of 

 this part of the apparatus, was provided with dark glasses, to 

 enable him, by watching the sun, to select the proper moment 

 and judge the length of time which each plate would require in 

 its exposure ; and he agrees with me in preferring this system 

 of operating to the use of a fixed mechanical contrivance for 

 opening and closing the aperture of the lens. 



By proceeding in the manner indicated, we endeavoured to re- 

 strict the photographic action to the representation of the sun's 

 disc alone, and only in the first and second plates of the series 

 was sufficient time allowed for the highly illuminated clouds 

 around the sun to become imprinted in the camera; although 

 faintly visible sometimes in the field of view of the telescope, their 

 intensity was now so much lowered as not to be copied. It will 

 be evident also from the foregoing description, that no clockwork 

 mechanism was required for the purpose of making the apparatus 

 follow the diurnal motion of the sun. 



The glass plates on which the pictures were taken measured 

 2| in. by 3| in., and were numbered at one corner by a scratch- 

 ing diamond ; they were cleaned previously and arranged in their 

 order of succession. The precise moment at which each plate re- 

 ceived its exposure in the camera was registered immediately ; 

 Greenwich mean time being taken the same morning from the 

 Woolwich Observatory, and kept by an ordinary good watch. 



The collodion process seemed on all accounts the most avail- 

 able, and was that employed to furnish the negative pictures from 

 which copies on paper have afterwards been printed ; and in order 

 to guard against accidents, the silver baths, collodion, and the 

 more important solutions were provided in duplicate, so that no 

 difficulty was experienced in preparing the sensitive plates in 

 rapid succession. The services of two of my pupils in photo- 



