1870.] The Solar Eclipse of August last. 41 



for the partial phases. During totality the full aperture of the 

 object-glass was employed, and a slide plate was used, having a 

 circular opening which allowed the full beam to pass. This plate 

 had two falls instead of one. On setting the plate free by the top 

 trigger it fell, and the collodion plate was exposed to the entire 

 beam ; after the desired exposure a lower trigger was relieved, and 

 the plate made a second fall, and the lens being covered by the top 

 of the plate, the exposure ceased. These triggers were connected 

 with a Morse register having a paper fillet running through it ; at 

 every second the clock for an instant opened the electric circuit, 

 and there was a very short break made in the line marked by the 

 pen on the fillet ; thus the seconds of time were stepped off in space 

 on the paper ribbon. The triggers were so connected with this 

 chronograph that an additional break was made during the time 

 the photographic plate was being exposed. By measuring this 

 break on the paper ribbon and comparing its length with the 

 length of the second in which it occurs, the exact fraction of the 

 second during which the plate was exposed will be given. 



Dr. Mayer arranged for his own duty to keep the telescope 

 in adjustment, and to manipulate the apparatus of exposure and 

 chronographic registration, while Mr. Willard placed the plate in 

 the camera and gave the several times of exposure he desired during 

 totality. Mr. Phillips coated the plates and handed them to Mr. 

 Montford, who carried them to Mr. Willard, and thence, after 

 exposure, to Mr. Mahoney, who developed them, assisted by Mr. 

 Leisenring. 



The wall of the dark room adjoining where the telescope stood 

 was fitted with two dark valves, or dumb waiters, by which the 

 plate-holders could be passed in and out without the admission of 

 light or the necessity of any of the operators moving from their 

 places. Seven negative baths were used, standing in a trough of 

 water to keep them cool, four plate-holders, and a large wooden 

 trough with grooved sides, similar to a negative-rack; this was 

 filled with a weak solution of hyposulphite of soda. In the dark 

 room the first operator's duty was to coat plates and - put them into 

 the baths ; the second took them out, put them into the plate-holders, 

 and passed them out of the room by means of one of the dumb 

 waiters. After exposure, the holders were returned to the dark 

 room by the second dumb waiter, when the third operator took the 

 plate from the holder, developed, washed, and then dropped it into 

 one of the grooves in the large fixing trough. There the plates 

 remained slowly fixing till after the eclipse was over, when they 

 were taken out in the same order in which they were put in, 

 washed, and numbered with a diamond. 



At the telescope Mr. Eock was detailed to attend to the very 

 important duty of calling out the seconds of the chronograph-fillet ; 



