286 
Scientific Notes . [April, 
their outline is as irregular as possible, light passes through them almost as 
though they were thin slices with parallel polished surfaces. This enables us 
to study the external staining, internal structure, and optical characters to 
great advantage, since they are not interfered with by any dark shading due 
to the bending of the light out of the line of vision. When examined in water 
there is no difficulty in recognising extremely minute granules of the kaolin of 
clays, whereas, when mounted in balsam, they may be almost or quite invisible, 
but this circumstance is of great advantage in observing certain fads, since, by 
makingthem invisible, other objeds may be distidly seen which otherwise would 
be completely hidden by the surrounding granules. 
Mr. Sorby mounts loose sandy deposits in the following manner: — Having 
placed a very small quantity of dissolved gum on the glass plate, the requisite 
amount of the deposit is taken and mixed with the gum, and sufficient water 
to make it easy to separate the grains and spread them uniformly over the 
space, which will be afterwards covered with the thin glass. The water is 
then allowed to evaporate slowly, and though much of the gum colleds round 
the margin, by properly regulating the quantity originally added, enough 
remains under the larger grains to hold them so fast that they are not squeezed 
out with the excess of balsam. More gum than is sufficient for this purpose 
should not be used, since it may make itself too conspicuous on the objed. When 
the proper quantity has been used its presence can be detected only at the under- 
surface of the grains, and in that situation does not in any way interfere with the 
study of the objed. This method prevents the grains from settling to one side of 
the objedt, even when soft balsam is used, which is desirable, since it pene- 
trates more completely at a lower temperature into irregularities of the surface, 
and into the interior of compound grains. In these observations, the employ- 
ment of objedt glasses of large angular aperture is disadvantageous, and with 
the higher powers it is impossible to focus down into the interior of grains of 
sand so as to view their minute fluid cavities. For these researches Mr. Sorby 
has had an eighth of only 75 0 of aperture constructed by Messrs. Beck, with which 
all parts of the objedt can easily be reached and perfect definition obtained with a 
power of about 600 linear; the form of grains as small as £ 0 ioo °f an inch * n 
diameter can also be seen. In the case of very fine transparent particles of 
such substances as pumice, the form is best seen when the illuminating pencil 
is rendered divergent by means of a conclave lens, instead of employing the 
usual condenser. This mode is, however, only available for the lower powers, 
on account of the light being somewhat feeble. A considerable portion of the 
paper is occupied with deta.led information respecting the examination of 
various fragmentary rocks and the means employed for their identification. 
Considerable use is made of the polariscope, as in the examination of rock 
sections. The paper is full of matter of the utmost value to the micro- 
mineralogist and geologist. 
Light. — M. Bert has undertaken some new experiments upon the influence 
of different colours upon vegetation. These experiments, performed chiefly 
upon the sensitive plant, lead to the following results : — Green light kills 
plants ; plants submitted to the influence of the green ray die in a short time. 
Under the influence of the red rays the sprays become elongated ; the leaflets 
are raised so as to form a smaller angle with the branch than in the normal 
state; the plant appears to become etiolated and yet it remains alive. Under 
the influence of the blue rays the process is reversed, the leaflets become 
perpendicular to the branch, whilst in white light an intermediate position is 
maintained, i.e., the leaflets form with the branch an angle of 45 0 on one side 
and of 75 0 on the other. M. Bert explains these fads as follows: — At the 
level of the point of attachment of the leaflet there is a motor enlargement, 
which increases or lessens in force according to the different kinds of rays. 
Under the influence of red rays there is formed in these enlargements a par- 
ticular substance, osmotic, and capable of attrading water. This substance 
generally disappears under the influence of blue rays. If we place it under a 
glass shade, red on one side and green on the other, the plant turns its leaflets 
towards the green, that is to say, towards the colour which kills it, and in fad 
it dies. 
