Solar Eclipse of May 28, 1900. 373 



The end of the tube was closed by a block of wood having an aperture 

 8 inches long by 3 inches wide, a little above the middle. Outside 

 this a long slide was arranged, at right angles to the telescope, and 

 bolted at the upper end to two stay rods attached to the telescope near 

 the mirror. 



A plate holder, 3 feet long by 10 inches wide, taking two plates 8| 

 inches square and four plates 8| by 4| inches, was arranged to move 

 along the slide by means of rackwork and a pinion wheel. One 

 revolution of the pinion moved the plates 2'13 inches, whereby four 

 images could be obtained on the square plates and two on each of 

 the narrow ones ; the sixteen images all being equal distances apart 

 and symmetrically placed on the plates. The revolutions of the pinion 

 wheel were controlled by a spring catch acting on the crank handle, 

 and holding it firmly in position after each revolution. 



The whole slide, carrying the plate holder, &c., was attached to the 

 telescope in such a way that the distance of the plates from the mirror 

 could be varied a small amount for focussing. 



The tube of the instrument was firmly bolted down to the sloping 

 side of a solid pier of stone and cement, built up within the observing 

 hut near the north end. It was adjusted so that the plane of disper- 

 sion of the prisms was in a meridian passing through the ccelostat, 

 and inclined to the prime meridian 68 (the hour angle of the sun at 

 mid-eclipse). The dispersion was therefore in a north and south direc- 

 tion. The internal contacts were computed to occur near to the south 

 point of the sun, and on either side of it. The centre of the flash 

 spectrum arcs was therefore midway between the edges of the spectnim 

 in the photographs obtained at mid-eclipse. 



The 2-inch Prismatic Camera. 



This instrument was the same which I employed successfully at the 

 Indian eclipse in 1898, excepting that it was fitted with a specially 

 corrected lens of 47 inches focus instead of the visual objective pre- 

 viously used. The images were therefore on a somewhat larger scale, 

 and larger plates were used. 



The sliding plate holder, constructed on the same lines as the larger 

 instrument already described, was made to hold three plates, 6^ by 4 

 inches, placed lengthwise in the holder ; and the crank handle moving 

 the slide was arranged to stop at each half revolution, moving the 

 plates 1'12 inches between each exposure. 



The two 60 prisms of this instrument are made of specially 

 selected crown glass, and are exceptionally transparent for ultra-violet 

 rays. 



The total deviation of the two prisms being approximately equal to 

 that of the reflecting spectrograph (about 80) the tubes of the two in- 



VOL. LXVII. 2 E 



