42 THE ESSEX FIELD CLUB.
perhaps the oldest forest in existence, belonging to the coal period ; it is at
Partick, near Glasgow, and the stumps of the trees are still standing as shown in
the picture. He also exhibited slides in relation to "physiography," or what the
Germans call "Earth knowledge." Some of them illustrated the ways in which
rivers cut valleys through both soft earth and hard rocks.
Prof. Meldola then came to the use of photography in physics. He explained
the analysis and synthesis of white light, and spoke of the photographing of the
spectrum. He exhibited a solar spectrum with plenty of lines in it, which he had
taken when associated with Mr. Norman Lockyer some years ago. He also told
of Dr. H. W. Vogel's origination of orthochromatic photography in 1874, and
how that branch of photography has been extended by the researches of Mr. C.
H. Bothamley, technical organiser for the county of Somerset. He dealt with
the wave theory of light, the phenomena of interference, and stated that the first
scientific use of photography was made in 1803 by Dr. Thomas Young, in photo-
graphing Newton's rings at the Royal Institution. Lord Rayleigh has just been
photographing interference bands, applying the phenomena to the revealing of
the degree of approximation to truth of asserted truly plane glass surfaces. Some
of the results, lent to the lecturer by Lord Rayleigh, were exhibited on the screen.
The photographs had been taken by pure sodium light, and plates made ortho-
chromatic by ammoniacal cyanin.
The lecturer lastly spoke of the "inductoscript" taken upon photographic
plates without light, by the Rev. Professor Smith, of Oxford. Electricity of
rapidly alternating current was used ; a coin was connected with one terminal,
and a sensitised plate with the other, and the coin and plate were brought very
near to one another. Then all light being excluded, the current was switched on
and was continued for fifteen minutes. At the end of that time the plate was
developed, and there was an image of the coin. A striking feature of it was that
from every projection in the milling there was a brush of rays, as one sometimes
sees depicted round the sun in old paintings. Prints could also be copied by this
process. Prof. Meldola said that he could not explain the production of this
picture ; the thing was not understood, it had yet to be investigated.
The Chairman, Mr. T. V. Holmes, in moving a vote of thanks to the lecturer,
remarked that Prof. Meldola was well aware of the extent to which photography
was being utilised by the British Association, because he was the Chairman of the
Committee of the Corresponding Societies connected with the Association, of
which societies the Essex Field Club was one of the chief. He hoped that its
members would help to supply the photographs the Association required. The
value of photography in geology was very considerable, and in two instances
which had occurred lately within his own knowledge in Essex, he had regretted
the absence of such a truthful record of observation. In surveying the railway
cutting between Upminster and Romford for his paper in the Essex Naturalist
(vol. vii., p. 1), he had found Boulder-Clay three miles farther south than had
previously been known. He had taken several geologists to the spot in order
that they might see the evidence of the fact with their own eyes, but a good
photograph would have been a sufficient record without further trouble. Again,
in the Romford cutting of the same railway, he had seen evidence of the existence
of an old river-bed, but a day or two later, when revisiting the spot with some
geological friends, the ground had been sloped, and in the absence of photographs
all chance of showing his friends what he had seen on former visits had vanished.
The river at Wakefield had been favoured in the photograph shown by Prof.