170 



NATURE 



[June 24, 1X97 



length of one of these rings was measured to be 5315 ; 

 it is due to the green corona line (1474K). The second 

 was stated to be coincident with D3. 



In 1883 the same instrument used in Egypt in 1882 

 was employed, as well as a 6-inch achromatic telescope, 

 and a concave Rowland grating of 5 feet focus, arranged 

 for taking ring spectra in the first and second orders. 



It is stated in the Report {Phil. Trans.^ 1889 A, vol. 

 clxxx. p. 122) that the photographs "possess no features 

 of interest," and neither reproductions, nor drawings nor 

 measurements are given. 



The prismatic camera employed in the eclipses of 

 1882 and 1883 was again used in the West Indies in 

 1886. Only the spectra of some prominences seem to 

 have been recorded. There is no mention of rings. 

 The hydrogen lines as well as K and / are noted {Phil. 

 Trans., 1889 A, vol. clxxx. p. 319). 



While on the one hand the photographic results, to 

 which reference has been made, certainly did not come 

 up to the expectations raised by my observations of 1871 ; 

 on the other, subsequent solar investigations confirmed 

 my opinion that this was the best way of studying the 

 lower parts of the sun's atmosphere, provided an instru- 

 ment of much greater light-grasping power could be 

 employed. 



I determined, therefore, when arranging for the 

 observations to be made during the eclipse of 1893, to 

 renew the attack with the largest telescope and the 

 greatest dispersion at my command. 



The Solar Physics Committee was then in possession 

 of a prismatic camera of 6 inches aperture. I decided, 

 therefore, to employ it, all the more because the work 

 on stellar spectra at Kensington had given abundant 

 proof of its excellence. 



The Eclipse of 1893. 



The instrument was entrusted to Mr. Fowler, the 

 demonstrator of astronomical physics in the Royal 

 College of Science, who erected it at Fundium in West 

 Africa, and obtained a series of photographs of the 

 greatest value to science. A greater success has never 

 been achieved in eclipse observations. 



The object-glass of this instrument, corrected for the 

 photographic rays, was constructed by the Brothers 

 Henry. The correction is such that it is unnecessary 

 to incline the back of the camera, and hence some of 

 the objections which have been made to the use of this 

 form of spectroscope are overcome. The large refract- 

 ing angle of the prism (45°) obviously increases the value 

 of the instrument for eclipse work. 



The camera has a focal length of 7 feet 6 inches, and 

 the spectrum obtained is about 2 inches long from F to 

 K. Rings corresponding to the inner corona are about 

 seven-eighths of an inch in diameter. 



The tube is a strong mahogany one, square in section, 

 and it was attached to the declination axis by means of 

 a suitable iron plate. In order to reduce the weight of 

 the instrumental equipment, the heavy iron pillar of the 

 equatorial was replaced by a rough wooden stand which 

 was filled up with concrete after being placed in position. 

 Provision was made for the clock bracket and fine 

 adjustments of the polar axis, and the whole arrangement 

 was quite satisfactory. 



Fig. 7 represents the instrument as adjusted for use in 

 latitude 14^ 3' N. When actually at work, the camera was 

 steadied by a stiff wooden rod screwed to the end of the 

 tube, and bearing on the end of the declination axis ; 

 this did not interfere with the driving gear and materially 

 contributed to the successful results, as on account of the 

 great weight of the prism it was necessary to bring a 

 large part of the tube forward to the eye end. The 

 brass cap which protected the camera from light other 

 than that which passed through the prism and object- 

 glass, is not shown in the diagram. 



NO. 1443. VOL. 56] 



As time is very precious during an eclipse, every effort 

 must be made to economise it. I may therefore refer to 

 the manner in which the photographic operations were 

 facilitated by the dark slides used. 



The construction of the camera and dark slides, or 

 plate-holders, was based on the plan which I devised for 

 the large pictures of the corona which I hoped to obtain 

 in the West Indies in 1886. The slides are about 13 

 inches in length by 7 inches broad, and have three com- 

 partments, each taking a plate 6 inches by 4 inches. 



The camera at the end of the long wooden tube has 

 an opening 6 inches square, and a rectangular frame 24 

 inches long, with a central aperture 6 inches by 4 inches, 

 and provided with grooves to take the slides, was sym- 

 metrically attached to it. A dark slide being placed in 

 the frame, so that the first compartment was opposite the 

 middle of the telescope tube, the shutter was then opened 

 to its full extent, and an exposure made ; the plate in the 

 second compartment was next brought to the middle of 

 the frame, by pushing the slide along, and also exposed ; 

 again, by moving the slide along, the third plate was 

 brought into position and exposed, after which the shutter 

 was closed and the slide withdrawn. During the ex- 

 posure of any one of the three plates in a slide, the other 

 two were protected from light by the rectangular frame. 



Fig. 7.— The 6-inch prismatic camera arranged for work during the Eclipse 

 of 1893. 



The upper edge of each dark slide was notched in 

 three places corresponding to the positions of the three 

 plates which it contained, and, as each plate came to the 

 proper position for exposure, as the slide was pushed 

 along, a spring catch automatically dropped into its 

 place. 



Upon the back of each dark slide six numbers were 

 painted in clear white figures. A small series of numbers 

 corresponded to the numbering of the thirty plates to be 

 exposed during the eclipse, and a larger series indicated 

 the exposures to be given to each plate, so that it was 

 unnecessary to refer to any list. 



These time-saving devices are of the highest import- 

 ance in eclipse work, and too much attention cannot be 

 given to them. The arrangements in West Africa 

 worked admirably, and it was possible to change from 

 one plate to another in about a second when a slide 

 was once inserted, and to change the whole slide in five 

 seconds. Longer intervals, however, were allowed to 

 elapse between the exposures, in order that the instru- 

 ment might steady itself, and to correct the backlash 

 of the driving screw. 



