M.uini 1. 1900.] 



KNOWLEDGE. 



51 



Mr. T. W. Baokhoiiso and Dr. Irwin Shar]3, at Biixar, 

 and by myself at Talni, by comparing it witli tlic 

 evening twilight illumination. 



Spectrosoopic obsei-\'a*ions of all sorts will be made. 

 from those with giant slit spectroscopes or object-glass 

 prisms to those with the humble but portable prismatic 

 opera-glass. Observei-s with the latter will probably 

 confine themselves to th3 shape of the Corona as seen 

 in the green corouium ring, the red and blue hydrogen 

 and the yellow helium rings. To attain the same 

 object without a spectres -ipic apparatus, Mr. Shackle- 

 ton suggests that it would be well to photograph the 

 corona by light as nearly iiionochroniatic as possible, 

 obtaining this pai-tly by using a film of special colour 

 sensibility and partly by the use of a colour screen. 



There are two classes of work that have been very 

 generally omitted from systvmatic observation in recent 

 eclipses. These are the ob^srvations of the shadow bands, 

 of which very little is as yc* known, and the visual study 

 of a small portion of the inner corona. The latter is 

 especially important for the true undei-standing of the 

 curious formations that in photographs appear to en- 

 wrap the brighter prominences and which lie at the 

 base of the great rays and the polar plumes. Only a 

 very small portion should be attempted by any one 

 observer since the time of totality is very short, but 

 the study of that portion should be thorough. 



Not very many English observers will seek the 

 American portion of the shadow track, and for these 

 there is only one method of reaching their station, 

 i.e., by a voyage across the Atlantic. For the eastern 

 observers there are many ways of travelling. By sea, 

 English astronomers can join the British Astronomical 

 Association's expedition, or go by regular lines to 

 Bordeaux, Santander, Lisbon, Cadiz, Gibraltar, Malaga, 

 or Algiers, from which ports they may travel inland by 

 rail to the central line, or .'n the case of Algiers observe 

 the eclipse at the port itself. For those wfto shirk 

 the Bay of Biscay, it is possible to travel overland 

 through Paris direct to Madrid from whence several 

 railway lines cross the path of shadow, or, by taking 

 ship at Marseilles, to cross the Mediterranean to 

 Algiers. For the overland travellers, the chief difficul- 

 ties will be in the transport of their instruments, and 

 113 the number of frontiers (which rpell custom-houses 

 and. perchance, disaster to photographic plates') which 

 must be crossed. 



ELECTRIC AUTO-PORTRAITS. 



By Alex. Thurburn. 



Since Mr. Brown, of Belfast, described the figures 

 he obtained by electric action on photographic dry 

 plates in " The London, Edinburgh, and Dublin Philo- 

 sophical Magazine " for December, 1888, accounts of 

 similar experiments with plates have from time to time 

 appeared, of which by far the most striking are those 

 contained in Lord Armstrong's fascinating and mag- 

 nificently illustrated book, " Electric Movement in Ai»- 

 and Water." I am, however, not aware of any such 

 experiments having been tried with sensitized papers, 

 although they yield interesting results, and suggest 

 various problems, when the figures thus produced are 

 compared with the corresponding ones on plates. If two 

 photographic plates are placed back to back between 

 the terminals of a discharger connected to a sufficiently 

 powerful induction coil, and a single discharge is made 

 to strike on them, and the platea are then developed 



the image produced on the plate which faced the 

 ])ositive terminal resembles in size, but is different i.i 

 form, from that produced on the plate which faced 



Flo. 1. — Noniiiil po.sitive on glajss. 



the negative terminal. The latter can be easily re- 

 cognised by the frond-like markings which form so 

 large a part of it, as may be seen in the illustrations 

 1 and 2. But if, instead of glass plates, we substitute 

 sensitized papers with a piece of glass between them to 

 prevent their being pierced by the spark, the negative 

 figure will b© much smaller than the positive, and will 



Fia. 



-Nununl iicf;iiti\i- ou glass. 



contain no trace of fronds. Such a negative is shown 

 bv Fig. 3. The coi-responding positive figure was nearly 

 three times larger in diameter, and the difference in 



