BLUE-VIOLET LIGHT IN THE SOLAR CORONA ON AUGUST 30, 1905. 309 



glasses, and when the armature is released the shutter turns again 45 degrees, as far 

 as one of the stops c, and shuts off the light. The contacts are made by a pendulum 

 clock, and they are so devised that make or break can occur only when the pendulum 

 is at or near its position of rest. 



I arranged for five exposures each of 1 second duration, and five exposures lasting 

 respectively 3, 9, 20, 46 and 89 seconds. Their actual durations are 0'84, 0'80, 078, 

 0'80, 0'85, 2'82, 9'02, 20'84, 45'91 and 89'04 seconds, as determined automatically on 

 the chronograph at the Observatory after my return from the eclipse expedition. I 

 have deducted 0'02 second from the figures recorded on the chronograph to allow 

 for the peculiar motion of the shutter. At the first four exposures of 1 second, 

 different screens, each with 13 holes, are in front of the object-glass. The diameters 

 of the openings are respectively 0'210, 0'296, 0'410, 0'595 inch. At the first exposure 

 the screen leaves 1/2T4 of the object-glass free, at the second 1/10-8, at the third 

 1/5-6, and at the fourth 1/27. These screens are geared to the clockwork which 

 rotates the shutter and fall out of gear after the fourth exposure. The illustration 

 shows them out of gear. 



The plate-holder (C) of the Cooke camera is 17x3 inches; it slides lengthways 

 inside a metal box 32 x 4 inches. It is moved by rack and pinion, the rack being 

 attached to the plate-holder, and the bearings of the axle of the pinion to the cover 

 (C,) of the box. Spring-driven clockwork (B) communicates its motion by means of a 

 shaft (a) to the pinion. The clockwork is governed (at b) by the armature of an 

 electro-magnet (the armature and the revolving stop with its axle appear white in 

 fig. 1). When the armature is attracted, the plate-holder moves 1 inch onwards, and 

 when it is released it moves another inch. The necessary contacts are made by the 

 pendulum clock. I arranged the contacts in such a way that for the first four 

 exposures the plate moves one step onwards, for all the others two steps, and when 

 the plate has l>een pushed along, 2 seconds are allowed for the camera to settle 

 before the next exposure is made. Of the 206 seconds for which I made provision, 

 173 seconds are occupied by the exposures, 15 seconds are taken up by changing 

 of plates, and 18 seconds are lost. 



The pendulum clock is shown at A. It is provided with four circular steel-sheet 



discs, into which notches are cut. The axle which carries the discs has a period of 



240 seconds, i.e., about half a minute more than totality lasted. Two of the discs 



(A, B) are represented in the diagram, fig. 2, which 



J ~"~"xi eiU a ^ 80 shows the contact levers. The diagram gives 



"'A ^: k-|l' __ the position immediately before making of contact. 



At the next second a will fall on b, making 

 contact at c, and after another second b will fall 

 away from a, thus breaking the contact. The 



duration of contacts depends slightly on the position of the notches, as will lie seen 

 from the figures given above for a second's contact, which show a range of 0'07 second. 



