112 PROGRESS OF MICROSCOPICAL SCIENCE. [7o"uSi,TuTriff 
paper on this subject, accompanied by numerous woodcuts. The 
following are descriptions of some of the more remarkable spectra 
given by a number of choice opals : — No. 1 gives a single black band 
in the red. When properly in focus this has a spiral structure. 
Examined with both eyes it appears in decided relief, and the arrange- 
ment of light and shade is such as to produce a striking resemblance 
to a twisted column. No. 2 gives an irregular line in the orange. 
Viewed binocularly, this exhibits the spiral structure in a marked 
manner, the different depths and distances standing well out ; upon 
turning the milled head of the stage-adjustment, so as to carry the 
opal slowly from left to right, the spiral line is seen to revolve and 
roll over, altering its shape and position in the spectrum. It is not 
easy to retain the conviction that one is looking merely at a band of 
deficient light in the spectrum, and not at a solid body, possessing 
dimensions and in actual motion. No. 3 gives a line between the 
yellow and green, vanishing to a point at the top, and near the bottom 
having a loop, in the centre of which the green appears. Higher up, 
in the green, is a broad green band, indistinct on one side and branch- 
ing out in different parts. No. 4 gives a broad, indistinct, and sloping 
band in the blue, and another, still more indistinct, in the violet. 
No. 5 gives a band in the yellow, not very sharp on one side, and 
somewhat sloping. Upon moving the opal sideways, it moves about 
from one part of the yellow field to another. In one position it 
covers the line D, and is opaque to the sodium-flame of a spirit-lamp. 
No. 6 gives a curiously shaped band in the red, very sharp and black, 
and terminating in one part at the line D. In the yellow there is a 
black dot. The spectrum of this opal showed by reflected light 
intensely bright red bands, of the shape of the transmission bands. 
On examining this opal with a power of 1 inch, in the ordinary 
manner, the portion giving this spectrum appeared to glow with 
intense red light, and was bounded with a tolerably definite outline. 
Without altering any other part of the microscope, the prisms were 
then pushed in so as to look at the whole surface of the opal through 
the prisms, but without the slit. The shape and appearance of the 
red patch were almost unaltered ; and here and there over other parts 
of the opal were seen little patches of homogeneous light, which, not 
having been fanned out by the prisms, retained their original slaape 
and appearance. No. 7 gives a black patch in the red, only extending 
a little distance, and a line in the yellow. On moving the opal the 
line in the red vanishes, and the other line changes its position and 
form. No. 8 gives the most striking example of a spiral rotating line 
which I have yet met with. On moving the opal sideways the line 
was seen to start from the red and roll over, like an irregularly shaped 
and somewhat hazy corkscrew, into the middle of the yellow. 
Effects of Induction-currents on Amoeba diffluens and Arcella vulgaris. 
— Some curious experiments have been made on these animals by M. 
T. W. Engelmann, and were recently reported to the Eoyal Academy 
of Sciences of Amsterdam. The animals were placed in a gas- 
chamber contrived with electrodes for the purpose, and the results 
were watched with the microscope. The following are some of the 
