1875.) The Late Transit of Venus. 185 
contact, and thus the observed moment of conta¢ét depends— 
to the extent of three or four seconds at least—on the ob- 
server’s idiosyncrasies. This equation may be estimated by 
practice on models; but it must always remain doubtful how 
far, in the excitement of transit observation, the personal 
equation remains the same as in the calm of ‘‘ model 
transit ’’ operations. 
Heliometric measurements, again, seem inferior to the 
instantaneous work of photography. On any reasonable 
assumption as to the skill of the observer, it is impossible to 
believe that he can measure the position of Venus with an 
accuracy comparable to that with which the photographic 
picture can be measured, if only that picture is clearly de- 
fined, and not affected by imperfections such as would render 
the process of measurement uncertain. For instance, specks 
and stains on the photograph do no harm. A contraction of 
the film in photographs on glass would be mischievous, as 
also would be the effects of so-called photographic irradia- 
tion, if measurement depended on the size of the photo- 
graphic image of the sun as affording a scale of measurement, 
while of course any optical defects would be fatal. But if 
such sources of error as these last can be in any way 
avoided, then photography must take its rank as facvle 
princeps among the available methods for dealing with transits 
of Venus. 
Now here, again, we approach one of those delicate 
questions unfortunately so numerous in the history of recent 
transit operations. English official astronomers were con- 
tent to take the opinion of Mr. De la Rue, a skilful photo- 
grapher of celestial objects, doubtless, but in the first place 
likely to view this matter solely in its photographic aspect, 
and secondly (as is manifest by his own account of the 
qualities of the method he advocated), unaware of the exact 
astronomical requirements for success. In this method, that 
used in the Kew Observatory, the image formed at the focus 
of the object-glass is optically enlarged before it is received 
on the photographic plate, and consequently its proportions 
depend on the instrumental adjustment, su that the only 
means of determining the scale of the sun picture is by a 
reference to the picture itself. But if there is any photo- 
graphic irradiation or shrinkage of the collodion film, the 
sun image will be affected ; in other words, the scale of mea- 
surement will be falsified. Now it is asserted by nearly every 
one who is at once competent to pronounce on photographic 
matters and acquainted with the mathematical aspect of 
the question, that there is no such accuracy in the outlines 
