88 
NATURE 
[Vov. 18, 1869 
without this construction, have rendered nice observation im- 
possible. : f 
The series of angles of the prism which I have given appears 
to me well adapted to general wants. I propose to furnish each 
of the principal telescopes to be used for the transit of Venus 
with a complete series of such prisms, arranged perhaps on a 
long slider. Care must be taken to make the thickness of the 
slider-frame as small as possible, inasmuch as it must be accom- 
panied with another slider carrying dark glasses. It will pro- 
bably be found best to place both sliders between the two glasses 
of the eye-piece. This slightly disturbs the elements of the 
calculation above ; but in practice the selection of the best prism 
will always be matter of trial, and the disturbance of calculations 
will be unimportant. 
Before closing this subject I will advert to a remark made by 
one of the most acute telescope-observers who have ever been 
known in this Society, the late Rev. W. R. Dawes. He states 
that, in general, a telescope performs better with one particular 
point of the edge of its object-glass upwards than in any other 
position. The explanation of this singular remark will be found, 
T think, in the combination of the effect of error of centering of 
the two lenses of an achromatic object-glass, with the effect of 
atmospheric dispersion. The centre of one lens (using the word 
“*centre ” to denote that part in which the tangent-planes of the 
two surfaces are parallel) ought to be exactly above the centre of 
the other lens. But it is not easy to make this adjustment per- 
fect ; the centre of one lens is frequently above a part of the 
other lens where the two surfaces have a slight inclination ; and 
the refraction thus created produces in the imagé of every star a 
spectrum which rotates as the telescope-tube 1s made to rotate. 
In one position of the tube the atmospheric dispersion is opposed 
to this, and may wholly or in a great measure correct it ; in the 
opposite position the atmospheric dispersion is added to it, and 
increases its injurious effects. 
The atmospheric dispersion between B and G is about {jth 
of the atmospheric refraction. At zenith-distance 45° it is nearly 
1’, at 63° it is nearly 2”, at 8o° about 5”. These are the lengths 
of the visible spectrum. 
The Cause of the Incandescence of Meteors 
THEincandescence of meteors wasat first ascribed to their friction 
against the air, until in 1854 M. Regnault showed that this was 
not probable. M. Goyi, of Turin, now affirms that the high 
temperature is due to the heat disengaged by the compression 
exercised on the air in front of them. This accounts for the fact 
that the interior of a meteor sometimes shows no signs of exces- 
sive heating, and that the hydrogen is not expelled. —[Bul. Asso- 
ciation Scientifique de France, t. vi. 305.] 
AMONG the points of interest touched upon at the last meeting 
of the Royal Astronomical Society was the extension to the ap- 
proaching transit of Venus of Professor Young’s suggestion to 
observe times of contact in solar eclipses by means of the gradual 
reduction of the length of the lines of the chromosphere, as 
observed in a spectroscope. 
BOTANY 
Spontaneous Motion of Protoplasm 
Pror. J. B. SCHNETZLER records in the Archives des Sciences 
Physiques et Naturelles, some observations on the spontaneous 
motion of the protoplasm in the cells of the leaves of the common 
water-weed, the Axacharis alsinastrum. The writer remarks 
that whether the cause of the motion is found, as some have 
maintained, in the successive contractions or vibrations of the 
exterior layer of the protoplasm, which transmit themselves to 
the interior layers; or whether the successive displacements of 
the molecules is produced by causes purely mechanical, as others 
have held, it still remains to be explained what produces these 
contractions or displacements. It is incontestable that they are 
found only in living protoplasm. Prof. Schnetzler believes that the 
principal cause which proyokes the motion is the chemical 
action of oxygen, which passes through the wall of the cell, and 
of which a portion is probably transformed into ozone under the 
influence of light, as occurs also in the globules of blood. The 
most strongly refracted rays of light have a marked influence 
on these currents, which are also no doubt affected by the 
currents of electricity which form, under the influence of water, 
between the surface of the leaf and the contents of the cells. 
The energy of the motion depends principally on the temperature, 
showing the greatest vigour between 16° and 20° C. In the 
point of view of mechanical theory, we have here evidently 
an example of the transformation of light and of heat into 
motion. The Axacharis is especially favourable for the obser- 
vation of these motions; as, in consequence of the transparency 
of its tissue, they can be watched under the microscope without 
any preparation. 
THE Lucerne crops in several parts of the country have recently 
been attacked by a species of Dodder, the Cuscuta hassiaca, 
allied to the parasitic Clover-dodder and Flax-dodder, which 
are so destructive to those crops. It is described as being a 
beautiful plant, with clear orange leafless stems, and abundant 
pure white and exquisitely-scented flowers. 
THE magnificent ‘‘ Flora Crasiliensis,” the magnum opus ot 
the late Von Martius, published under the auspices of the Bavarian 
and Brazilian Governments, is not likely to suffer by the death of 
that distinguished botanist. Under the able* editorship of 
Dr. Eichler, of Miinich, two new parts have recently been 
published, a most valuable and beautifully illustrated dissertation 
on the curious parasitic Balanophore by the Editor, and a 
monograph of the Brazilian Convolvudacee by the veteran Meissner. 
In the course of the ensuing winter we are promised a yolume on 
the Ferns, about 350 species, with nature-printed illustrations, by 
Mr. J. G. Baker, of the Kew Herbarium ; and the most eminent 
European botanists are engaged on other orders which still 
remain to complete the work. AL Wi Bs 
CHEMISTRY 
Italian Mineral Waters 
THE following analyses of Italian Mineral Waters have been 
made by Prof. Purgotti of Perugia [Ann. di Chim, app. July, 
1869, p. 59.] ; ) 
I. Bromo-ioduretted water which collects in a reservoir, five 
kilometres from the station of Assisi :— 
Carbon dioxide . 
0°44110 
Silica SEs "01500 
Magnesium bromide 000124 
Magnesium chloride 0°18830 
Magnesium sulphydrate . 0°07750 
Sodium chloride . 0°86370 
Sodium sulphate 5 0°15630 
Calcium bicarbonate . 0°35800 
Magnesium bicarbonate . 025190 
Extractive organic matters . 0°02150 
‘Total mineral constituents - 2737454 
WERE ho 6.15 907°62546 
100000000 
This water likewise contained free oxygen, ozone, ferrous 
bicarbonate, and alumina (and iodine ?), but in quantities too 
small for estimation. 
II. A ferruginous water collected in a square reservoir about 
half a kilometre from Cannara, near Collemancio, was found to 
contain :— 
Ferrous bicarbonate 0'0300 grm, 
Manganous bicarbonate 0°0036 
Free carbon dioxide 
Atmospheric air Silica 
Calcium bicarbonate Calcium sulphate 
The temperature of this water is considerably lower than that 
of the surrounding air. 
Sal-ammoniacum Martiale 
ANGELO Baniert has made the following observations on the 
ammonio-ferric sulphate (sa/ ammoniacum martiale) collected on 
the lava of Vesuvius. Many naturalists believe that the hydro- 
chloric acid eyolyed by lavas in their course, unites with the iron 
of the same lavas, forming ferric chloride, which, together with the 
ammonia of the air, gives rise to the compound of sal-ammoniac 
and ferric chloride found in the fumaroles. This view, however, 
does not appear to the author to be in harmony with facts 
observed in the Vesuvian lava-current of 1850. It was only in that 
part of the laya which had overwhelmed a cultivated and manured 
soil that fumaroles existed, and there they were so numerous as to 
yield more than 100 measured quintals of sal-ammoniac, whereas, — 
on the other part of the igneous current, which had passed over an 
older lava of the year 1834, in which there was nothing but dry 
rock and sterile sand, there were no fumaroles of sal-ammoniac. 
The silica of the lavas acts at very high temperatures on the 
common salt contained in the manured soil, liberating hydro-— 
chloric acid, which, on the one hand, reacts on the ferric hydrate 
Magnesium bicarbonate 
Magnesium chloride 
Magnesium sulphate 
