The Past and Coming Transits and Arctic Explorations. 313 



chord from the sun's centre is what is really required. Now, setting 

 aside the heliometic method, by which it was hoped that the distance 

 of Venus from the sun's centre might be accurately determined, 

 photography promised a means of indicating the required element very 

 satisfactorily, because a solar photograph is secured in the fraction of 

 a second, and ample means exist for indicating the precise instant at 

 which each photograph is taken. 



In a paper, the geometrical principles indicated in which were 

 professedly adopted by Government astronomers, in the choice of 

 stations for photographing the late transit, I showed how the difficulty 

 of indicating the exact angle of position of the line joining the centre 

 of the disc might be obviated, and every thing made to depend on the 

 measurement of the distance between the centres, assuming the longi- 

 tude of the station known, and the exact instant of each photographic 

 record assigned. It did not seem to me necessary to point out, that, 

 as the time of mid-transit drew near, the effect of any time-error 

 (whether in the indication of the instant of exposure, or in the deter- 

 mination of the longitude of the station) would be diminished ; for 

 this is a fact, not only obvious in itself, but taken for granted in the 

 discussion of the whole matter by all who have considered the 

 geometrical relations involved. So that I took it as self-evident that 

 mid-transit was the time when photographic records for determining 

 the chord of transit would have greatest value. And it was easy to 

 perceive that, in some cases it might be advantageous to select stations, 

 either solely or chiefly with reference to the important phase of mid- 

 transit; in other words, to select stations where neither the beginning 

 nor the end of transit could be photographed under favorable condi- 

 tions, but where the middle of transit would be most advantageously 

 observable. However, when this obvious particular case of general 

 theory I had dealt with was pointed out by Mr. E. L. Garbett, I 

 presented the suggestion as though it contained somewhat of novelty, 

 not caring at that time to show how completely it was included in the 

 general reasoning advanced by Colonel Tennant, Dr. De la Rue, and, 

 more fully by myself.* 



On the occasion of the late transit only one station was specially 

 suited for mid-transit photography— Cape Town. Though Natal 

 would have been worth occupying. Cape Town was superior to every 

 other southern station for this particular purpose. But somehow the 

 suggestion that photographs should be secured there was overlooked, 



* Nor should 1 aow caH attention to the point but that the special form ofthanks adopted by 

 Mr. Uarb.tt for what, in reality was unnecessary generosity, consisted Iri denuncumons 

 addressed t.. mo for overlooking tUe point which had appeared to myself and others too obvioua 

 for special mcution. 



