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Taytor—On the Photographing of the Solar Corona. 273 
The instruments previously used had been of comparatively 
small aperture and of moderate focal length, the limits of six 
inches aperture, and six or seven feet focus being very rarely 
exceeded. Professor Pickering, at the eclipse in California, 1889, 
January 1, used a photographic object-glass of 13 inches aperture 
and 16 feet focus (the longest focal length up to that time), and 
Father Perry, at the eclipse at Salut Isles, French Cayenne, 1889, 
December 21-22, used a mirror of 20 inches aperture and 40 in. 
focus; but these were exceptional instruments. 
In 1893, April 16, four stations were occupied, and the eclipse 
was successfully photographed at all of these. Professor W. H. 
Pickering was at Minasaras, in Chili, and used a 5-inch object- 
glass of about 48 inches focus, stopped down to 3 inches aperture, 
and a 20-inch mirror of 45 inches focus. Professor Schoeberle, of 
the Lick Observatory, was at Mina Bronces, in Chili, and he 
and his assistant, Mr. W. F. Gale, of New South Wales, used 
two instruments, one of 4:96 inches aperture, and 46 feet focus, 
and the other of 64 inches aperture and 6 feet focus. At Para 
Curu, in Brazil, I used two optical combinations, one a 4-inch 
photographic lens of 60 inches focus, and the other a 4-inch lens 
of about the same focus, but fitted with a triple cemented negative 
enlarger of 8 inches negative focus, which enlarged the image 
3 diameters, thus corresponding to 4 inches aperture and 15 feet 
focus. At Fundium, in West Africa, Serjeant J. Kearney, R.E., 
used a double camera exactly similar to mine in Brazil. Comte 
de la Baume Pluvinel, of Paris, who was also at Fundium, West 
Africa, used nine different objectives (mounted on one stand), 
all of sensibly the same focus, 13 metres (4 feet 10 inches), and 
with apertures varying from 155 mm. to 5 mm. Other instru- 
ments were used at the various stations, but these are the chief 
ones, and those from which valuable scientific results were obtained. 
The principal departures from ordinary practice were the long 
focal-length instruments, 46 feet in Chili, and 15 feet in Brazil 
and Africa, and the exceptionally small apertures, 5 mm., of Comte 
de la Baume Pluvinel, in Africa. 
One of the most disputed points in eclipse photography has 
been as to the proper exposure to obtain the faint extensions of 
the corona without fogging the plate by the skylight. Captain 
Abney has shown that we may look upon a photograph as repre- 
senting 200 different shades; or, in other words, that on a 
