SCIENTIFIC SUMMARY. 
101 
Saturn came nearest to tlie sun in August, 1841, and Jupiter in February, 
1839, and also that they were closer about that period than at the previous 
and following aphelion of Saturn. As two revolutions of Saturn are equal to 
nearly five of Jupiter, it would follow that the same phenomenon would occur 
every fifty-nine years, which he thinks agrees very well with Professor Wolf’s 
period of fifty-six years, and that the epoch of 1840 is not far distant from 
1836, as found by him. 
Transit Ohservations.—'M.Y. Dunkin has made an interesting comparison 
of the accuracy of the old method of observing the passages of stars as 
effected by the eye and ear, and the modern one in use at the Greenwich 
Observatory, in which the observation is recorded by the galvanic or chrono- 
graphic method. He finds that by the latter method the minute fractions of 
time by which the most practised observers differ from each other is very 
constant, whilst by the former it was variable ; the difference between two 
observers who had been engaged together for many years, varying from one- 
third to two-thirds of a second in the course of seven years. He finds that 
the probable error of a transit by the old plan 0'029 seconds, whilst with the 
new it is only 0’017 seconds. With the former it would appear, that with 
slow-moving stars the accuracy of the observer’s judgment decreased, whilst 
it remained constant with the latter. The probable error of the right 
ascension of a star was 0’048 seconds by the eye and ear, and 0’034 seconds 
by the eye and touch. The smallness of those errors may be imagined, as 
Mr. Pritchard observed, by placing a human hair at a distance of 125 feet, 
when its thickness would be 0'022 seconds of time. The cause of the dis- 
cordance between observers with the galvanic method lies in the fact that the 
finger may move a little too soon or too late on the tappet, and imperfect 
galvanic contact take place. Professor Wheatstone proposes that this should 
be remedied by a moveable wire following the star as it crossed the field, and 
when it came in contact with the fixed ones that the chronographic contact 
would be made, and thus a number of records obtained independent of the 
will of the observer. A method similar to this was employed by Mr. De la 
Rue in his lunar-photographic experiments. According to Colonel Strange, 
the suggestion of Professor Wheatstone has already been carried into effect 
by M. Redier, at the Paris Observatory, with perfect success, the wire being 
carried across the field so steadily and so exactly, with the same velocity as 
the passage of the star, that the intersection was perfectly certain. It would 
appear that the French astronomers are, however, somewhat favourable to 
the old system, although not so far as to exclude the new method, M. Le 
V errier believing “ that a touch observer does not acquire the same apprecia- 
tion of duration of time as an observer trained to eye and ear observations.” 
Nebula of Orion . — It having been noticed by the Greenwich observers 
that the drawing of this nebula, as given by Sir J. Herschel in 1836, was 
more accurate than that lately published by Professor Bond, the latter cele- 
brated observer examines this statement, and points out the discrepancies in 
detail between the two drawings, in which he considers his own to be the 
more correct. In Herschel’s delineation he points out the following discre- 
pancies : — (1) The absence of a definite limit to the bright light of the 
Huygenian region on its eastern side, close to the southern shore of the 
Sinus Magnus, and its consequent extension along this shore to a distance of 
