Total Solar Eclipse of 1900 (May 28). 357 



object glass, by means of which an image of the sun can be thrown 

 upon the slit. 



The whole arrangement thus consists of a spectroscope combined 

 with a polar heliostat, and in virtue of the fact that the spectroscope 

 is rotated together with the mirror, the image of any celestial object 

 thrown upon the slit does not rotate relatively to the slit. Further- 

 more, the mirror is mounted in such a manner that the axis about 

 which it can be tilted namely, the declination axis can be oriented 

 relatively to the collimator tube, so that any diameter of the sun may 

 be set parallel to the slit. 



A special plate holder was designed for use in Algiers in order to 

 facilitate tlie rapid change of plates. It was charged with twelve 

 plates, fixed film outwards on the outside of a cylinder (2 inches in 

 diameter), whose axis was set parallel to the focal plane of the camera 

 and in the plane of dispersion, free to turn inside a slightly larger 

 covering cylindrical case. The arrangement was turned by hand, and 

 worked admirably well. It is, however, only suitable for narrow 

 spectrum plates, and might be used with very small alteration for a film 

 on celluloid, such as is used in hand cameras of the Kodak type. 



The linear dispersion in the photographed spectrum is about 14 tenth- 

 metres per millimetre at H y . The width of the slit was adjusted to 

 O03 mm. by a diffractional method. 



The scale of the photograph is such that one degree on the sky 

 corresponds to about 9 mm. on the plate. 



The effective aperture of the combination regarded as an instrument 

 for producing monochromatic images of a slit-shaped region of the 

 corona is// 10. 



The adjustment of the axis of the instrument to parallelism with the 

 earth's axis was accomplished in the same way as in India by means 

 of a theodolite with declination circle and level, which was attached to 

 a part of the frame of the spectroscope specially prepared for it. 



Programme of exposures, &c. : 



I. Spectrum of the Sun's Limb at the Beginning of Totality. Five 

 exposures were made in 7 seconds, beginning 3 seconds before Professor 

 Turner's signal " Start " was called, and ending as Mr. Wyles called 

 the " fifth " beat of the metronome, 



Result. The developed photographs show that the first plate was 

 exposed at exactly the right moment to catch the spectrum of the 

 " flash." It is filled with bright lines, and shows the part of the 

 spectrum between Hf (3900) arid H^ (4861). The best part of the 

 spectrum is that between wave-lengths 4100 and 4650. 



All the other four plates show bright lines, but the fall in the 

 number of them is very abrupt between the first and the second plates. 



II. Spectrum of the Corona. Six seconds after Professor Turner's 

 signal " Start " a plate was exposed for the spectrum of the corona, and 



VOL. LXVII. 2 D 



