TOTAL ECLIPSE OF THE SUN, APRIL 16, 1893. 
561 
Conditions of the Eclipse in Africa. 
The results obtained with the prismatic camera will vary in detail according to the 
conditions of eclipse, and it is therefore desirable to indicate the conditions obtaining 
in 1893. 
At the African station, the apparent diameter of the sun and moon were respec¬ 
tively 31' 55' ‘4 and 33' 35"‘9 ; the altitude at the time of eclipse was about 53°. The 
first contact, according to the ‘ Nautical Almanac ’ Circular, took place at an angle of 
about 130°, reckoned from the North point towards the West, and the last contact at 
an angle of 57° East of the North point. Hence, at the commencement of totality 
the contact would occur about 60° East of the North point, and at the end of totality 
about 128° W. 
A calculation for the relative movement of the moon and sun during totality gives 
0"'37 per second. On this relative movement depends the length of time during 
which the chromosphere will be visible after the beginning and before the end of 
totality; thus, if the chromosphere were 5" in depth, it would have been visible for 
about 13 seconds on this particular occasion. The apparent depth of the chromosphere 
seen at the commencement of totality will be diminishing at the rate of 0"’37 per 
second, while that seen near the end of totality, on the opposite limb, will be increasing 
at the same rate. The spectroscopic appearances in successive photographs vary 
accordingly. 
The duration of totality calculated for a place very near the selected station was 
4 minutes 12'4 seconds (‘Nautical Almanac’ Circular, No. 14). As measured by 
M. Coculesco, a member of the French Expedition at Fundium, who was specially 
occupied with this question, the duration was 4 minutes 11 seconds.* 
A consideration of the conditions of the eclipse indicated that on the whole the 
best position for the prism was that giving the dispersion in a North and South line ; 
so long as the direction of dispersion is not nearly tangential to the sun at the points 
of contact, it matters little what is its direction as regards the photographs taken 
during totality. For the photographs out of totality, where the object is to study 
the phenomena at the cusps, the direction of dispersion must be such that the spectra 
of the two cusps are not superposed, and, if other circumstances permit, the best 
position of dispersion would be perpendicular to the line joining the cusps. 
With an equatorially mounted prismatic camera, the position of the prism giving 
dispersion in a North and South line is by far the most convenient for practically 
working the instrument, as the deviation of the prism can be readily corrected by a 
movement in declination alone, and photographs of stellar spectra can be taken for 
focussing pui'poses. Hence, for the sake of greater simplicity in working, it was 
decided to work with a North and South dispersion, although this involved a sacrifice 
MDCCCXCVI.—A. 
* ‘ Comptes Rendus,’ 1893, vol. 116, p. 1238. 
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