392 KANSAS CITY REVIEW OF SCIENCE. 



very simple and easy, there are some decided drawbacks to its successful prose- 

 cution. It is comparatively easy to determine the time of external contact both 

 when Venus enters and when she leaves the disc of the Sun. But to determine 

 the exact time when the body of the planet has just fully entered the disc, and 

 when it is just about to begin its passage across the edge at egress is an entirely 

 different matter. The dense atmosphere of Venus surrounds the planet with a 

 rim of hazy light and this, with the defraction and irradiation necessary under 

 the circumstances, renders the determination of the exact time of geometrical 

 contact impossible. So great is the effect of these causes that two observers, us- 

 ing similar instruments, and watching side by side, may differ as much as a min- 

 ute in their estimate of the time of the occurrence. The one redeeming feature 

 of the method is that it requires a difference of two seconds of time to make an 

 error of so much as one-tenth of a second of arc in the measurement, so slow is 

 the motion of Venus across the face of the Sun. And it may be that, by proper- 

 ly training the observers and instructing them carefully about the appearances to 

 be seen, the method may be made to yield satisfactory results. It would seem, 

 then, that the English, with their large outlay, are running considerable risk in 

 trusting entirely to this one method. 



The second method is essentially German in its history and use. The in- 

 strument — the heliometer, — with which alone the measurements can be satisfac- 

 torily made, has done its best work in German hands, and it is but right and 

 natural that they should use it to its full capacity. They do not, however, in- 

 tend to confine themselves to this one method but will also take observations of 

 the times of contacts. 



In the photographic method the observations are made almost entirely by 

 mechanical means and thus the personal equation, that fertile source of discrep- 

 ancies, is almost entirely obliterated. The instruments required, though expen- 

 sive and delicate, are not cumbrous, the work itself is not essentially difiRcult, 

 and there is the decided advantage of a permanent record that can be referred to 

 at any time. It is claimed that the collodion films of the photographic plates are 

 liable to slip and render the record worthless; but, as it seems, this statement has 

 yet to be proved. The principal reason why this method is not to be used more 

 generally at the coming transit is said to be that, because the photographic ob- 

 servations made in 1874 have not yet been completely reduced and their worth 

 fully shown, European astronomers do not wish to try an improved method but 

 prefer to use those whose errors they know. Be this as it may, the results al- 

 ready obtained from a partial discussion of the observations show a probable er- 

 ror far less than those obtained by any of the other processes. May it not be 

 that our cousins of England and Germany are just a little chagrined about the 

 fact that their photographic work of eight years ago was decidedly inferior to 

 that done by the Americans ? As the case stands at present, the greatest diffi- 

 culty in the way of a successful result of the labors of the present year is the lack 

 of harmony between the masters of the science as to best methods of observation. 

 Even if advisable, it is a difficult matter to combine the results found by different 



