114 



Dr. W. Huggins. 



corona had really been obtained upon the plates. On one plate 

 especially forms resembling closely in character those present in the 

 photographs of the eclipse of that year were visible. 



In the following year, 1883, working with the Lassell mirror, I 

 found that when the sky was free from clouds, but whity from a 

 strong scattering of the sun's light, the sun's image in the photographs 

 was well defined upon a sensibly uniform surrounding of air-glare, 

 but without any such sudden increase of illumination near the sun's 

 limb, or other indication which might be due to the corona. It was 

 only when the sky was exceptionally clear that coronal appearances 

 presented themselves with more or less distinctness. 



use more than 3^ inches of the central portion of the speculum, partly for the 

 reason that a larger amount of light would be difficult of management, and partly 

 because this restriction of the aperture would enable me to adopt the arrangement 

 which is shown in the diagram. 



" It will be seen at once from an inspection of the diagram that in this arrange- 

 ment the disadvantage of a second reflection by the small mirror is avoided, as is 

 also the mechanical inconvenience of tilting the speculum within the tube as in 

 the ordinary form of the Herschelian telescope. The speculum b remains in its 

 place at the end of the tube a, d. 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,/, 3i inches in diameter, for the light to enter ; 

 below is a similar hole, over which is fitted a framework to receive the ' backs ' con- 

 taining the photographic plates,, and also to receive a frame with fine ground glass, 

 for putting the apparatus into position. Immediately below, towards 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 / is fixed a tube, <?, 6 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 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 f 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 2 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, 2 inches in 

 7 feet, does not affect sensibly the performance of the mirror. The whole apparatus 

 is firmly strapped on to the refractor of the equatorial, and carried with it by the 

 clock motion." — Report B. Ass. Advanc. Science, 1883 p. 346. 



