52 Capt. E. H. Hills and Mr. H. F. Newall. 



All the above plates were successfully developed the night after 

 the eclipse, and positive copies on glass were made to guard against 

 loss. 



A reproduction of one of the best photographs (No. 3, Dallmeyer 

 lens, 12 sees, exposure), is given in Plate 1 (frontispiece). 



IV. The Spectroscopic Cameras. 

 By Capt. Hills. 



' Instruments. The details of the two spectroscopes used were as 

 follows : 



Objective 



Collimator and camera 

 lenses. 



Slit . . . 

 Prisms 



Prisms at min. deviation 

 for 



Position of slit with re- 

 spect to sun's image. 



Spectroscope No. 1. 



Cooke achromatic, 4J in. 



aperture, 5 ft. 10 in. 



focus. 

 Single quartz lens, 2^ 



in. aperture, 30 in. 



focus. 



H in. by 0*0018 in. 

 Two dense flint prisms 



of 60, 4fc in. base, 



2| in. height. 



Parallel to meridian 

 through sun's centre, 

 cutting limb at point 

 of second contact. 



Spectroscope No. 2. 



Single quartz lens, 5 in. 



aperture, 4 ft. 9 in. 



focus. 

 Single quartz lens, 3 in. 



aperture, 36 in. focus. 



2 in. by 0'0014 in. 



Four double quartz 

 prisms of 60 (each 

 prism being composed 

 of two half-prisms of 

 right- and left-handed 

 quartz), 3 in. base, 2f 

 n. height. 



He. 



Vertically diametral. 



The slits were in each case adjusted to such a width as to realise 

 one-seventh of the theoretical maximum resolving power of the 

 prisms. In the case of spectroscope No. I which, as will be shortly 

 seen, was used for most of the work, this amounted to about 0'3 of 

 an Angstrom unit in the violet. It may be noted that any higher 

 degree of resolving power would have been wasted owing to the 

 coarseness of grain of the photographic plate, the above figure repre- 

 senting not only the calculated resolving power of the instrument, 

 but that actually realised on a trial plate. 



The length of the spectrum on the plate was 3J inches from H/s 

 toK. 



Both spectroscopes were mounted in an approximately horizontal 

 position, and were supplied with light by a heliostat, furnished with 

 a 12-inch flat mirror. 



