Dec. 29, 1881] ; 
NATURE 
207 
gins ; otherwise it would be difficult to explain how colder water 
might remain on the surface, were it not for the greater amount 
of salt in the lower strata. It was always difficult to explain, 
M. Woeikoff observes, how ice is formed on the surface of 
oceans, the temperature of the maximum of density being lower 
than that of the congelation ; but the question is complicated in 
oceans by many causes, and therefore M. Woeikoff asked M. 
Liston to make observations on a salt lake : but Lake Kupalnoye 
contains too much salt to be compared with oceanic water, and 
thus it would be desirable to make experiments on this subject on 
large tanks filled with salt water, and exposed during the winter 
in rough climates to the faction of low temperatures and of 
radiation. 
In the last number of the Geneva Archives des Sciences Naturelles, 
M. W. Meyer gives an account of the applications of the micro- 
telephone in the Observatory of Geneva for the transmission of 
the beats of a clock to observers occupied in different rooms of 
the Observatory. The experiments have been carried on for 
eighteen months, and although at first there were many difficul- 
ties, satisfactory results have been reached. During last year 
all comparisons of clocks were mide by means of the micro- 
telephone, with the same accuracy as if they had been made 
directly. The average error of each observation does not exceed 
0'06 of a second, and theconstant error, deduced from fifty-nine 
series of observations, is very small (o‘001 of a second). An- 
other very interestins experiment was made in connection with 
the determination of longitudes between Vienna and Geneva, 
which was undertaken by MM. Oppolzer and Plantamour. By 
a new application of the microphone, M. Meyer caused the beats 
of the Observatory clock to be inscribed automatically on a 
chronograph ; and afterwards, by putting this clock into com. 
munication with the Vienna Observatory, he made it inscribe its 
beats automatically on both chronographs, at Vienna and at 
Geneva, avoiding thus the auxiliary movement which usually 
establishes the contact in electrical clocks, and may be a cause 
of inaccuracy. MM. Plantamour and Oppolzer, being both at 
the time in Vienna, were able to ascertain the accuracy of this 
new combination, and they afterwards made use of it for the 
comparison of the Geneva clock with the electrical chronometer. 
A few improvements in the microphone add very much to the 
accuracy of the signals and the ease with which they are trans- 
mitted to the chronograph. 
On the sensitive surface of the body small spaces (it is known) 
can be distinguished within which two or more stimuli appear 
as one or simple; these vary in size and form in different parts. 
The retina must here be included. Now, the rods and cones 
forming the mosaic of the retina were recently counted by Herr 
Salzer, and in the central and most sensitive part—the yellow 
spot, where are only cones—he found a huadreuth of a square 
millimetre to contain 132 to 138 of these. Dr. Du Bois Rey- 
mond has lately tested by experiment the suppvsition that these 
elements correspond to the ‘‘ circles of sensibility” for the retina, 
If so, the number of lizht-points which, on the same extent of 
retinal surface (‘OI sq. mm.) give separate visual impressions, 
should correspoud to Salzer’s number of cones. The manner of 
experiment was this: The observer looked through a tube, 
blackened interiorly towards a perforated screen which could be 
shifted in the line of a beam of reflected sunlight coming towards 
the eye. The screen had, in a fraue, a piece of tinfoil 5 ctm. 
square, perforated regularly with a fine sewing-needle in 460 
places. This, while looked at, was gradually withdrawn in the 
line of sight, A point is reached, at which the lizht-points tend 
to unite ia short lines ; with further removal the lines are con- 
tinuous, as in a grating ; and with still further, the distinction of 
lines is lost. The distances corresponding to such effects were 
noted, a..d again, in bringing the screen back, The results are 
considered to confirm the view under trial, viz. that the number of 
circles of sensibility in the yellow spot is equal to the number of 
cones. When there were seventy-four light-points (or half the 
number of cones) in oor sq. mm., they could just be distinguished, 
and beyond 149 the lines disappeared. 
In a recent number of Waturven, Prof. Axel Blytt concludes 
the highly interesting series of papers in which he has at some 
length expounded his theory of the immigrations into Norway of 
different floras during early dry and wet periods. On carefully 
examining the oldest Norwegian turf bogs, he finds, as Prof. 
Steenstrup has shown in Denmark, that four distinct turf layers 
may be traced between which there are frequently two, or even 
three, equally distinct deposits, composed of the roots and other 
remains of trees. ‘The latter are found zx sécw, and by the un- 
disturbed condition of the turf-beds above and below them they 
afford a conclu-ive proof that such severed trunks cannot have 
been cut down by human agency. These separate tree beds the 
author regards as mementoes of long periods of dryness, which 
may have endured for thousands of years, and during which the 
formation of turf was arrested, to be resumed again when a wet 
period supervened. Such interrupted periods of dryness and 
wet he considers to be closely related to the several long inter- 
rupted glacial periods, which, according to Geikie, have suc- 
ceeded one another. In accordance with Herr Blytt’s view the 
close of the first glacial age was followed by a dry period in 
which an Arctic flora appeared in Scandinavia, traces of which, 
as leaves of Dryas octopetala and Salix reticulata, have been 
found in the clay underlying the bogs in Denmark and Southern 
Sweden, in the latter of which the same flora is to be seen inter- 
posed between two ancient moraines. The boreal flora, the 
author is of opinion, we may refer to a dry period, charac- 
terised by great summer heat ; and in the deposits belonging to 
this age we find abundant remains of such deciduous trees as, 
e.g. the hazel and the Prunus avium, which are now of rare occur- 
rence in Norway, while many other vegetable forms represented 
in these beds have been long extinct. The differences observable 
in the bogs of Denmark and Norway Herr Blytt refers to the 
fact that while the former has undergone very little if any altera- 
tion of elevation, the latter has risen since the glacial age 600 
feet above the level ofthe sea. In Norway the formation of the 
turf beds may be gauzed by their varying elevations. Thus in 
South-East Norway, where the old sea-level has been raised to 
a height of 600 feet, the turf is from 20 to 26 feet deep, while 
at low levels, as 30 feet above the strand, the bogs are seldom 
more than from 2 to 4 feet deep. The author believes that 
we are justified in expecting that a more careful working out of 
the theory of the alternation in early times of dry and wet periods 
will help to elucidate many hitherto unexplained geological and 
botanical relations, including the distribution of plants; and he 
considers it probable that the temperature of the ocean, on which 
climate so largely depends, may similarly be subjected to periodic 
changes dependent on cosmical Jaws not less firmly fixed than 
those which control the movements of the planetary worlds. 
We have received from the editor of the Natural History 
Yournal various forms for the entry of observatiuns on meteoro- 
logy and natural history, which are issued to their correspon- 
dents indifferent parts of the country, the meteorological forms 
being returned to Mr. J. E. Clark, 20, Bootham, York, while 
those referring to natural history are communicated to Mr. F. A. 
Lees, Wetherby, Yorkshire. The proposed meteorological ob- 
servations are fairly satisfactory as regards the natural pheno- 
mena in connection with which they are made. The mean date 
of flowering of each of the thirty selected wild plapts for the 
last three years is given, a feature in the forms well calculated 
to awaken and sustain the interest of the observers. By these 
observatious carefully made and recorded from ‘year to year 
