Aug. 25, 1879] 
NATURE 
343 
period of the first flowering of plants, and the first appearance of 
migratory birds, insects, and other animals of the summer season. 
THE last volume (xrx.) of the Transactions of the Imperial 
Zoological-Botanical Society of Vienna contains, among its 
more important articles, contributions to the flora of Greece 
and Crete, by Dr. E. Weiss; a monograph of the genus 
Botrychium, by Dr. J. Milde (reducing the thirteen species in 
Moore’s Index Filicum to ten); anatomical investigation of 
Pleurophyllidia formosa, by R. Bergh; a second contribution to 
the flora of Lower Austria, by Dr. A. Neilreich; observa- 
tions on the metamorphosis of insects in the light of the theory 
of descent, by F. Brauer (a thoroughly Darwinian article) ; 
contributions to Hymenopterology, by Dr. J. Kriechbaumer ; 
descriptions of several Myriapods in the Museum of Vienna, by 
MM. A. Humbert and H. de Saussure ; the Lichens of the 
‘Tyrol, by F. Arnold; zoological notes, by G. Ritter yon 
Frauenfeld ; contributions to the Fish-fauna of Trans-baikal, 
by B. N. Dybowski. 
In the ‘‘ Arbeiten aus der Kiel Institut” we observe that Kliinder 
has been making further investigations into the time occupied in 
muscular contraction. His experiments have been conducted 
with a pendulum chronoscope constructed by Hensen. The 
contraction is traced on a reddened glass plate attached to the 
arm of a tuning fork, with which it therefore vibrates when this 
is sounded. ‘The curve described is consequently a sinuous line, 
its ascending and descending portion decussating. Ifa vertical 
median line be drawn on the plate when at rest, the measurements 
can be examined and compared. These give for the stage of 
latent excitation a value of z%,ths of a second, which, when the 
muscle is weighted or exhausted, may rise to more than o’OI sec. 
Antecedent extension diminishes the duration of this period, 
as Helmholtz had already remarked. ‘The proper curve of con- 
traction exhibits itself in its middle part as a curved line modified 
by the elasticity of the muscle. The muscle is quite inactive 
towards the end of contraction, as shown by the form of the ex- 
tremities of the curve. The greatest increase in rapidity occurs 
in the ascending portion of the curve, which corresponds to the 
greatest development of force in the muscle which is between 
the 3rd and 4th 1-400 of a sec., the absolute greatest rapidity 
of the ascent isin the 8th 1-400 sec. The form of the curve 
is, considerably changed if a heavy weight is appended to the 
muscle, the period of elevation as well as the fall being both 
longer. The retardation occurs principally at the commence- 
ment of the elevation, at which period the rapidity only slowly 
increases, as compared with its usual rate. 
THE COMING TRANSITS OF VENUS* 
‘T RANSITS of Venus over the disc of the sun have more than 
any other celestial phenomena occupied the attention and 
called forth the energies of the astronomical world. In the last 
century they furnished the only means known of learning the 
distance of the sun with an approach to accuracy, and were 
therefore looked for with an interest corresponding to the im- 
portance of this element. Although other methods of arriving 
at this knowledge with equal accuracy are now known, the rarity 
of the phenomenon in question insures for it an amount of atten- 
tion which no other system of observation can command. As 
the rival method, that of observations of Mars at favourable 
times, requires, equally with this, the general co-operation of 
astronomers, the power of securing this co-operation does in 
itself give the Transits of Venus an advantage they would not 
otherwise possess. 
Although the next transit does not occur for four years, the 
preliminary arrangements for its observation are already being 
_ made by the governmental and scientific organisations of Europe. 
* Substance of a paper read before the Thirteenth Annual Session of the 
American Academy of Sciences, held at Washington, by Prof. Simon New- 
comb. (The original paper was illustrated by diagrams.) 
Tt is not likely that our Government will be backward in fur- 
nishing the means to enable its astronomers to take part in this 
work, The principal dangers are, I apprehend, those of setting 
out with insufficient preparation, with unmatured plans of ob- 
servation, and without a good system of co-operation among the 
several parties. For this reason I beg leave to call the attention 
of the Academy to a discussion of the measures by which we 
may hope for an accurate result. 
In planning determinations of the solar parallax from the 
Transits of Venus, it has hitherto been the custom to depend 
entirely upon the observations of the internal contact of the 
limbs of the sun and planet proposed by Halley. It is a little 
remarkable, that while astronomical observations in general have 
attained a degree of accuracy wholly unthought of in the time of 
Halley, this particular observation has never been made with a 
precision at all approaching that which Halley believed that he 
himself had actually attained. In his paper he states that he 
was sure of the time of the internal contact of Mercury and the 
sun withina second. The latest observations of a transit of 
Mercury, made in November 1868, are, as we shall presently 
see, uncertain by several seconds. It is also well known that 
the observations of the last transit of Venus, that of June 1769, 
failed to fix the solar parallax with the certainty which was looked 
for, the result of the standard discussion being now known to be 
erroneous hy one-thirtieth of its entire amount. One of the first 
steps to carry out the object of the present paper will be an 
inquiry into the causes of this failure, and into the different views 
which have been held respecting it. 
The discrepancies which have always been found in the class 
of observations referred to, when the results of different ob- 
servers have been compared, has been generally attributed to the 
effect of irradiation. The phenomenon of irradiation presents 
itself in this form : When we view a bright body, projected upon 
a dark-ground, the apparent contour of the bright body projects 
beyond its actual contour. The highest phenomenal generalisa- 
tion of irradiation which I am aware of having been reached is 
this: A lucid point, however viewed, presents itself to the 
sense, not as a mathematical point, but as a surface of appre- 
ciable extent. A bright body being composed of an infinity of 
lucid points, its apparent enlargement is an evident result of the 
law just cited. 
[The speaker here drew a number of diagrams for the purpose 
of illustrating his theory. ] zy 
The following diagrams show the effect of this law upon the 
time of interval contact of a planet with the disc of the sun. 
The planet being supposed to approach the solar disc, Fig. 1 
shows the geometrical form of a portion of the apparent surface 
of the sun, or the phenomenon as it would be if there were no 
irradiation immediately before the moment of internal contact. 
Fig. 2 shows the corresponding appearance immediately after 
the contact. To indicate the effect of irradiation, or to show 
the phenomenon as it will actually appear on the theory of irradia- 
tion, we have only to draw an infinity of minute circles for each 
point of the sun’s disc visible around the planet to indicate the 
apparent phenomenon. The effect of this is shown in Figs. 1@ 
and 2a. The exceedingly thin thread shown in Fig. 1 is thus 
thickened as in 1a, and the sharp cusps of Fig. 2 are rounded 
off as shown in Fig, 2a. The apparent radius of the planet is 
diminished by an amount equal to the radius of the circle of 
irradiation, and the radius of the sun is increased by the same 
amount. Comparing Figs. Ia and 2a, it will be seen that the 
moment of internal contact is marked by the formation of a 
ligament, or ‘‘ black drop,” between the limbs of the sun and 
the planet. This formation is of so marked a charaeter that it 
has been generally supposed there could be little doubt of the 
moment of its occurrence. The remarks of the observers have 
given colour to this supposition, the black drop being generally 
described as appearing suddenly at a definite moment. 
Examining Fig. 2a, it will be seen that the planet still ap- 
pears entirely within the disc of the sun. The geometrical circle 
which bounds the latter, and that which bounds the planet, 
instead of touching, are separated by an amount equal to double 
the irradiation. And, when they finally do touch, neither of 
them will be visible at the point of contact. The estimate of 
the moment of contact must therefore be very rough, the means 
of estimating being far less accurate than those afforded by a 
common filar micrometer. In the actual case the eye has to con- 
tinue the two circles to the point of contact by estimation, 
through a distance depending on the amount of irradiation, 
while measures with a micrometer are made by actual contact 
of a wire with adisc. Such estimates have, therefore, been 
