312 The Past and Coming Transits and Arctic Explorations. 



Nor can photography be of any use in this matter ; for the more 

 perfect the photographic arrangements, the more exactly will the 

 optical difficulty be reproduced. Indeed, in the photographic records 

 of contact, during the recent transit, a peculiarity appears, which 

 seems, of itself to introduce an absolutely insuperable difficulty. It 

 would seem that the sun, which photographs itself, is slightly larger 

 than the sun we see ; in other words, that the gaseous matter of the 

 sun emits light waves, producing that form of chemical action on which 

 photography depends, from layers extending to a greater height than 

 those which emit light waves recognizable by the eye in full sunlight. 

 Janssen, at least, adopts this interpretation of the fact, that an ingress, 

 as observed at Nagasaki, the planet appeared still attached to the solar 

 limb, while the photographs taken, second by second, showed Venus 

 already somewhat advanced on the sun's disc. It matters in truth, 

 very little whether this explanation is correct or not, seeing that the 

 observed facts, however explained, indicate a discrepancy between the 

 optical and the photographic records of contact which must prevent 

 our placing reliance on either. If we abandon contact observa- 

 tions, but one resource seems to be left. It is manifest that all 

 methods have for their real object the determination of the chord of 

 transit followed by Venus, as seen from different stations. When 

 reliance was placed on Halley's method, for instance, although the 

 element observed was the duration of transit, the element deduced was 

 the length — and with the length the position — of the chord of transit. 

 When reliance was placed on Delisle's method, the element observed 

 was the epoch either of ingress or egress ; but the element deduced 

 was the position of the ingress or egress end of the chord of transit, and 

 therefore of that chord itself. 



The great difficulty in all other methods of determining the position 

 of the chord of transit resides in the fact that the exact position of 

 Venus on the sun's disc (not merely her distance from the center, but 

 her bearing from the center, referred to some fixed line on the sun's 

 disc,) must be determined for a precisely -timed moment. So that a 

 double difficulty is introduced ; first, the observations necessary to 

 determine Venus's position require time, secondly, the exact longitude 

 of the station should be known with as great accuracy as for Delisle's 

 method. But the central part of the chord of transit — the part, 

 namely, where the planet makes its nearest approach to the sun's 

 centre — has this advantage over the other parts : the planet's distance 

 changes so slowly as it is passing this portion, that an error of a few 

 seconds would be comparatively of small importance. Moreover, this 

 part of the chord is the most important because the distance of the 



