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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 he comparatively of small importance. Moreover, 
this part of the chord is the most important because the dis- 
tance of the chord from the sun’s centre is what is really 
required. Now, setting aside the heliometric 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 Grovernment 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 centres of the disc might be obviated, and 
everything made to depend on the measurement of the distance 
between the centres, assuming the longitude 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 determination 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 favourable conditions, but where the middle of transit 
would be most advantageously observable. However, when this 
obvious particular case of the general theory I had dealt with 
was pointed out by Mr. E. L. Grarbett, I presented the sugges- 
tion 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, Hr. De la Eue, and, 
more fully, by myself.* On the occasion of the late transit only 
* Nor should I now call attention to the point hut that the special form 
of thanks adopted by Mr. Garbett for what, in reality, was unnecessary 
