Solar Eclipse of May 28, 1900. 371 



occulted by the moon comparatively slowly. Much more time is 

 therefore available for taking a series of photographs than is the case 

 at stations near the central line of the eclipse, where the moon's 

 motion is at right angles to the layer, and opportunities for obtaining- 

 photographs of the very lowest strata are reduced to a fraction of a 

 second only at each internal contact. 



I decided therefore to choose some point situated well within the 

 zone of total eclipse, but so far from the central line that the two- 

 internal contacts would be separated by an angle of about 39 

 on the sun's limb. This would give a duration of totality equal 

 to one-third that at the nearest point on the central line ; and the 

 time available for photographing the flash spectrum would not be 

 less than 30 seconds. At mid-eclipse the moon's limb would overlap 

 the photosphere about 1", so that even at that time the flash spectrum 

 layer would not be entirely hidden. 



Under these circumstances, also, one of the contacts would take 

 place at, or very near to, one of the poles of the sun, the other being 

 in latitude 51. A succession of photographs taken during totality 

 would therefore give a series of images of the flash spectrum ranging 

 from solar latitude 51 to the pole. 



In selecting the most suitable station, dry ness of climate was con- 

 sidered to be the most important factor for securing extension of the 

 spectra in the ultra violet ; I therefore selected Algeria in preference 

 to Spain, although the altitude of the sun would be less in the former 

 country. 



The best position in Algeria for realising the greatest solar altitude 

 was a point on the coast west of Algiers, and on the southern border 

 of the eclipse track. This region was therefore decided upon at the 

 outset, and in order to realise the favourable conditions mentioned 

 above, two stations were selected provisionally beforehand, and for 

 these Dr. Downing kindly computed for me the durations of totality 

 according to the data used by the Nautical Almanac Office. 



The first station, near to the village of Zeralda, was found by him to 

 have a duration of 45 seconds, i.e., more than one-half the central line 

 duration. The other station, three miles further south, and near to- 

 Maelma, was computed to have a duration of 29 '5 seconds with a 

 possible error of 10 seconds. 



As the required conditions would, apparently, be very nearly fulfilled 

 at the latter station, I decided to place my camp either at that precise 

 spot, or at some point situated on a line passing through it, and 

 parallel to the direction of the shadow track in that region. 



The actual station eventually chosen was 6 '5 kilometres distant from 

 the station near Maelma, in a direction bearing West 25 North. Here it 

 was estimated that totality would last 30 seconds. Unfortunately, as 

 the event proved, the value of the diameter of the moon adopted by 



