1875.] the Observation of the Transit of Venus. 67 



worked either by hand or automatically by means of clockwork. This axis 

 also carries an ivory ring on the periphery of which is fixed a piece of 

 platinum wire which, as the axis revolves, comes into contact with a strip of 

 platinum fixed on a spring attached to a connector, so that it may be placed 

 in electrical communication with a chronograph and electric clock and thus 

 enable the precise moment to be recorded, when the uncovering of the aperture 

 in the shutter of the slide by the exposing disc exposes a portion of the plate 

 to the sun. As there are sixty slots and the aperture corresponds to the 

 sixtieth part of the circumference of the plate, it is evident that for each 

 entire revolution of the plate-holder sixty distinct images will be impres- 

 sed on as many separate portions of the plate within an annular space about 

 1 in. wide round its circumference. 



The apparatus is constructed so that the plane of the sensitive collodion 

 film shall exactly coincide with that of the focussing screen of the camera, 

 and in order to adjust the instrument so as to obtain an image of any de- 

 sired portion of the solar limb or disc, it is arranged that when the sensitive 

 plate is in the proper position for receiving the first image of the sixty, 

 the observer can look from behind, through a series of three red glasses, one 

 of which is in front of the plate, on the exposing disc, and the other two be- 

 hind it, one on the revolving plate-holder and the other on the wooden 

 case. The three glasses are coincident only in one position, i. e., when the 

 stop, formed by racking the last of the radial slots for only a short dis- 

 tance, is on the right of the axis ; and as the stop is on the left of the axis 

 after a complete revolution, the revolving plate-holder must always be 

 reversed through an entire revolution after each operation in order to bring it 

 into the proper position for focussing. While focussing, the sensitive plate 

 itself acts as a focussing screen. 



By means of clockwork the rate of revolution of the plate-holder could 

 be so adjusted that the exposures might be made at intervals varying from 

 about half a second to two seconds, but as it was desirable not to expose the 

 separate pictures too rapidly, the rate was set so that the entire revolution 

 might be accomplished in about a minute and a half. 



Preliminary trials with Dry Plates. — I received intimation about the 

 middle of August 1874 that, with the concurrence of the Surveyor General, my 

 services were likely to be placed at Colonel Tennant's disposal for the superin- 

 tendence of the photographic observations. As there appeared to be a general 

 opinion in Europe that a dry process would be most suitable for con- 

 tinuous observations lasting over a period of some hours and would 

 have other special advantages for the purpose, the first thing to be done 

 was to select the process to be used and to gain some experience in working 

 it ; and although the weather at that time of the year was most unfavorable 

 to photography and very trying to work in, all the time that could be 



