DOS 



PROFESSOR L. BECKER ON THE DISTRIBUTION OF 



Cooke triple achromatic lens of 3 inches aperture and 58 - 5 inches focal length, which 

 belongs to the Glasgow spectrograph, the other with a Ross portrait lens of 2 inches 

 aperture and 12 inches focal length. The pictures obtained with the larger camera 

 are so much superior to the small size ones of the portrait lens that I have not made 

 use of the latter in this paper. The cameras were fed by a coelostat of 8 inches 

 aperture, which had been kindly lent to me by the Royal Dublin Society. In front 



Fig. 1. 



of the two object-glasses, and about an inch from them, a rotating shutter was 

 mounted which served both cameras. The rotating shutter has four oblong apertures, 

 90 degrees apart (its back view is shown at D 2 , fig. 1) ; it is rotated by clockwork 

 driven by a spring, and its motion is governed by the armature of an electro- magnet (/). 

 When the armature is attracted, the shutter rotates through about 45 degrees until 

 it presses against one of the four stops d and brings an opening opposite the object- 



