APPARATUS FOR RAPID FORMATION OF PHOTOGRAPHS. (19) 



When the preparations for the Transit Expedition were far advanced, a proposal was published 

 by M. Janssen for taking a photograph of Venus and a portion of the Sun's limb near to it at 

 every second of time, or other short interval, near to the times of ingress or egress. It appeared 

 desirable to make trial of this proposal ; and, under my general superintendence, an apparatus 

 was prepared by Mr. Dallmeyer, with great skill, which appeared likely to carry out M. Janssen's 

 proposals perfectly well. A circular photographic glass plate, lOf inches in diameter, mounted 

 in a large ring whose circumference was cut in teeth, was so arranged that, when its frame was 

 fixed upon the photographic end of the photoheliograph, with the photographic plane transversal 

 to the optical axis of the instrument, it could be made to rotate by a small toothed-wheel spindle 

 (whose axis was parallel to the optical axis), but not continuously. During a portion of the 

 rotation of the spindle the large ring and photographic plate stood still ; and the plate was exposed 

 to the Sun, and an image was formed. (This was effected by cutting away some of the spindle- 

 teeth.) During the remaining portion of the rotation the sun-light was stopped, and the motion 

 of the spindle gave motion to the ring and plate so as to expose a new part for a new photo- 

 graphic image. The winch by which the spindle was turned was very near the center of motion 

 of the great tube ; and it was found that, when turned by a careful hand, no tremor could be dis- 

 covered in the photographic apparatus. The photographic images were to be measured by due 

 adaptation of the photograph-micrometer. 



The number of images thus taken and entered in Form 21 was: At Thebes, 83; at Rod- 

 riguez, 146 ; at Roorkee, 53. But they have not been further reduced. The ardour of the 

 Observers had been much cooled by the apparent general failure of the photographic principle, 

 and they were unwilling to spend further time on these reductions. 



1881, June 10. G. B. AIRY, 



4A 



