ON THE SOLAR CORONA. 759 



In the spring of the following year, 1883, the attack upon the co- 

 rona was carried on with a more suitable apparatus. The Misses Lassell 

 were kind enough to lend me a seven-foot Newtonian telescope made 

 by Mr. Lassell, which possesses great perfection of figure and retains 

 still its fine polish. For the purpose of avoiding the disadvantage of a 

 second reflection from the small mirror, and also of reducing the aper- 

 ture to three and a half inches, which gives a more manageable amount 

 of light, I adopted the arrangement of the instrument which is shown 

 in the following woodcut : 



K 



a 



ne 

 K 



2 a 



'SUNS I MACE 

 a \ ^SHUTTER 



TUBE WITH O/APHRAGM 



The speculum b remains in its place at the end of the tube a, a, 

 by which the mechanical inconvenience of tilting the speculum with- 

 in the tube as in the ordinary form of the Herschelian telescope is 

 avoided. 



The small plane speculum and the arm carrying it were removed. 

 The open end of the tube is fitted with a mahogany cover. In this 

 cover at one side is a circular hole,/! three and a quarter inches diame- 

 ter, for the light to enter ; below is a similar hole over which is fitted 

 a framework to receive the " backs " containing the photographic plates, 

 and also to receive a frame with fine-ground glass for putting the 

 apparatus into position. Immediately below, toward the speculum, is 

 fixed a shutter with an opening of adjustable width, which can be 

 made to pass across more or less rapidly by the use of India-rubber 

 bands of different degrees of strength. In front of the opening f is 

 fixed a tube, c, six feet long, fitted with diaphragms, to restrict as far as 

 possible the light which enters the telescope to that which comes from 

 the sun and the sky immediately around it. The telescope-tube a, , 

 is also fitted with diaphragms, which are not shown in the diagram, 

 to keep from the plate all light, except that coming directly from the 

 speculum. It is obvious that, when the sun's light entering the tube 

 at/" falls upon the central part of the speculum, the image of the sun 

 will be formed in the middle of the second opening at d, about two 

 inches from the position it would take if the tube were directed axially 

 to the sun. The exquisite definition of the photographic images of 

 the sun shows, as was to be expected, that this small deviation from 

 the axial direction, two inches in seven feet, does not affect sensibly 

 the performance of the mirror. The whole apparatus is firmly strapped 

 on to the refractor of the equatorial in my observatory, and carried 

 with it by the clock-motion. 



