JANUARY 25, 1901.] 
When the disk revolves, however rapidly, 
in the beam of sunlight parallel to its sur- 
face as stated, the eye regarding the inner 
surfaces of the rods a, b, c, d, e, sees the 
* usual sheen of light intersected with in- 
tensely black shadow bands due to f, 9, h, 7,7, 
projected in the positions a, 6,c,d,e. The 
experiment is specially striking in a dark 
room. This phenomenon is easily explained, 
for the points a, b, c—are nodal points, as 
it were, or loci of perpetual eclipse. 
The second phenomenon is even more 
striking: if the light after passing the re- 
volving disk parallel to its face (which 
therefore is seen as a mere line) is caught 
on a screen, either close at hand without a 
lens, or at long range by lens projection, the 
loci 2, 6,7, 5, «, are again stationary, ap- 
pearing however bright on a dark ground. 
Judged merely by the eye, the effect is just 
SCIENCE. oes 
volving rods. Hence more light must get 
through along the lines fe, gd, he, than in 
the same direction between these lines. 
C. Barus. 
BROWN UNIVERSITY. 
THE INTERNATIONAL GEODETIC 
ASSOCIATION. 
Tue sessions of the Thirteenth General 
Conference of the International Geodetic 
Association were held at Paris, France, be- 
tween the 25th of September and the 6th of 
October, 1900. 
The following countries were represented 
at the Conference: Germany (8), Austria 
(2), Denmark (1), Spain (1), United States 
(1), Great Britain (1), Hungary (1), Italy 
(3), Japan (1), Mexico (2), Norway (1), 
Holland (3), Roumania (2), Russia (1), 
Sweden (1), Switzerland (1), France (8), 
as if the rods where they cross in double 
thickness were perfectly transparent. It 
would take considerable acumen to predict 
this kinematic result. 
Without entering into details, I may 
point out in explanation that every part of 
the area of projection, S, is swept by the 
shadow of each rod twice per rotation. 
The result must be perceptible and at first 
sight uniform darkening of the field. But 
whereas throughout the whole semicircle, 
f, 9, h, —and also between the points a, b, c,— 
light has been removed backwards (i. ¢., 
from the screen) by reflection, none has 
been removed at the points a, 6, c; for these 
appear as black lines in the sheen of re- 
the number following the name of each 
country indicating the number of delegates 
sent from the country. Belgium, Greece 
and Portugal were not represented. 
At the opening session of the Conference 
the French Minister of Public Instruction 
presided, and welcomed the delegates in 
the name of the French Government. The 
sessions were held in the new Sorbonne, 
and a great many interesting reports were 
read. From these reports the following 
details have been extracted : 
M. Schumann has undertaken a new 
computation of all the great arcs already 
measured, in order to deduce the elements 
of the terrestrial ellipsoid. 
