374 Mr. -T. 



fitruments were arranged nearly parallel, the 2-inch spectrograph l>eing 

 screwed to the side of the reflector with its aperture alongside that of 

 the latter. The camera end with the sliding plate holder was at the 

 lower end. 



This was rigged up while in camp, as it was found that a small 

 portion of the coelostat mirror was available to supply light. It con- 

 sists of two double quartz prisms of 60 and 40 angle respectively, 

 each prism having 1^-inch square faces; and a single quartz lens of 

 24 inches focus. It was screwed on the top of the 2-inch spectrograph, 

 with its apertiire just within the elliptical beam of light from the 

 coelostat. 



My brother arranged a very convenient exposing shutter, which he 

 was to open near mid-eclipse for a single exposure of 10 seconds. 



Mrt]i<lx of Focussing. All three spectrographs were approximately 

 focussed by taking a series of photographs of the spectrum of Venus. 

 Having determined the focus of the reflecting prismatic camera in this 

 way within very narrow limits, I used this instrument as a collimator 

 for the 2-inch spectrograph, removing the large prisms, and adjusting 

 H slit in the position occupied by the plates when photographing 

 Venus. The 2-inch instrument was attached to the wall of the hut, 

 with its aperture inside the tube of the reflector, and directed towards 

 the mirror. With the north door of the hut widely open, the slit was 

 illumined by light from the sky, and a series of photographs was 

 obtained of the Fraunhofer spectrum. 



For some reason the focus was not so sharply defined, and the 

 definition of the lines was not so good as in photographs I had obtained 

 by the same method before leaving England. 



A third method was therefore tried. After having adjusted the 

 2-inch spectrograph in its correct place, photographs were obtained of 

 the sun itself on Sandell triple plates ; the focus being determined by 

 the images which were most sharply defined along the edges. 



A final method, which was adopted for the two larger instruments, 

 was to adjust the focus visually during the eclipse itself, using the 

 Fraunhofer lines, which become sharply defined shortly before totality. 



Programme of Exposures. The two larger prismatic cameras were 

 each to have sixteen exposures made simultaneously, by removing a 

 plate of aluminium from the common aperture of the two instruments. 

 The first, second, fifteenth, and sixteenth exposure were to be of 

 about 1 second duration each, and the remaining exposures of 2 seconds 

 duration, excepting the exposure nearest to mid-eclipse, which was to 

 have 10 seconds. 



The quartz prismatic camera was to be exposed near mid-eclipse also 

 for 10 seconds. 



