SCIENTIFIC SUMMAET. 
329 
and the circular or semicircular. Also cases in which the bottom of the 
channel presents longitudinally a sensible curvature — non-permanent move- 
ments, such as those in rivers at times of flood — the laws which regulate 
the propagation of waves at the surface of flowing water, with the influence 
of slopes, friction, curves, &c. The committee remark on the agreement 
between the author’s theoretical results and those of actual experience, and 
regard the treatise as one of much practical value. 
Can One Neutralise Irradiation^ — M. F. P. Le Roux states in the 
‘^Comptes Rendus” of April 14th, that he can at will neutralise irradiation. 
Thus, he can see the circumference of the luminous part of the moon con- 
tinuous with that of the dark part j or see a flame no longer encroach on a 
screen placed beside it ; the angles of white squares on a chess board no 
longer joined by a white ligament (indeed, the appearance may be reversed, 
and dark squares be seen joined by a dark ligament). He points out that 
for the field of distinct vision, about the fovea centralis^ there is no irradia- 
tion. From this spot outwards the distinctness of perception rapidly de- 
creases, and we naturally tend to equilibrate the sensations at the periphery 
of the object. If one of the horns of the lunar crescent is brought into the 
field of the fovea centralis^ the irradiation there disappears ; but by a peculiar 
compensation that at the other horn appears more than doubled. M. Le 
Roux further discusses the case of observation wdth the naked eye (as well 
as with telescopes when observing transits of Venus) of a dark ligament 
between two opaque bodies on an illuminated ground, and says that by 
sustained attention he can make these disappear. The ligament is not a 
phenomenon of irradiation, but of imperfect accommodation. 
The Absorption Bands of Chlorophyll have been also studied by M. Chau- 
tard, who (according to the ‘‘Chemical News” of June 12th) divides the 
chlorophyll bands into three distinct categories. The first contains simply 
the band in the middle of the red ; this he calls the specific hand. In the 
second he includes all bands which have been observed in chlorophyll solu- 
tions, new or old, neutral, acid, or alkaline ; these he calls supernumerary 
hands. The most remarkable is that which results from division of the 
specific band in the red, under the influence of alkalies. The third category 
comprises accidental handsj not having the permanent character of the pre- 
ceding, and being produced in special conditions. Of this kind is that from 
a division of the specific band through acids. The additional band here 
seems to arise from the less refrangible side, while in the alkaline solution it 
arises from the other. M. Chautard gives several particulars as to the 
method of treating chlorophyll in order to obtain various bands. 
Comparing Electrical Machines . — This has been done by M. Muscart, who 
says, in a paper before the French Academy, that an electric machine may 
be characterised aecording to two constants — (I) the difference of potential 
which it is capable of producing between two conductors 5 (2) the quantity 
of electricity which it can yield in a given time. The writer has made com- 
parison of eleven different kinds of machine, comprising three Ramsden, 
one Van Marum, one Nairne, three Holtz, one Carre (with caoutchouc 
plate), one Armstrong, and the induction coil. A table gives the diameter 
of the plate, the production per turn, and the production per second. 
A Spectral Illuminator is described in the “ Coniptes Rendus” and also 
