TOTAL ECLIPSE OF THE SUN, APRIL 16, 1S93. 
587 
photosphere, will be shown in each of its radiations which is bright enough to be 
photographed, as a ring brightest at the outer edge, and dimming very rapidly 
towards the photosphere in consequence of its greater thickness in the line of sight 
near the tangent. If only the outer part were bright enough to be photographed, 
the ring would have a diameter greater than that of the moon. Layers considerably 
removed from the photosphere would be cool and dim, and their feeble images would 
tend to be lost in the general continuous spectrum. The diameters of the rings 
being different, true wave-lengths could not be assigned, and the superposition of a 
great number of them would give the appearance of a nearly continuous spectrum, 
in the part lying within the band equal in breadth to the moon’s diameter. Outside 
this band, where the rings would be best visible, above and below the moon, on 
account of their different diameters, the direction of dispersion is such as to cause 
the greatest overlapping of images, and the consequent confusion would make the 
spectrum look nearly continuous, in the case of a considerable number of images. 
Hence there will be some difficulty in detecting the effects due to a very large 
number of concentric separate layers when the prismatic camera is employed during 
totality. 
The conditions with regard to layers in the photographs taken out of totality with 
the prismatic camera, are somewhat as follows :— 
If the vapours extend from the photosphere outwards, and are brightest at the 
base, the arcs due to them which appear at the cusps, provided they are bright 
Fig. 11. 
Possible appearances of bright arcs at cusp in photographs taken out of totality. 
(а) Arc due to vapour extending from photosphere outwards, with gradually diminishing brightness. 
(б) Arc due to a thin layer close to photosphere and equally bright throughout. 
(c) Arc due to a shell of vapour, concentric with photosphere, but some distance from it. 
(d) Continuous spectrum of photosphere. 
enough to show themselves in the general illumination of the field will have a some¬ 
what triangular appearance, with the maximum brightness nearest to the cusp as 
shown at a, in fig. 11. 
The intensity of such an arc will gradually diminish in all directions from the cusp 
and its extension will depend upon intrinsic brightness. All the arcs will have the 
same internal radius. 
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