208 
NATORL. 
[JunE 29, 1899 
and the results of their examination prove that it is possible to 
get large fields sensibly free from optical distortion. This has a 
most important bearing on the carrying out of the ‘‘ chart” 
work, as it is at present necessary to expose for one hour to 
obtain a region 2° x 2°, whereas the new form of lens would 
give a much larger region in the same time. For this reason, 
Prof. Turner has indefinitely postponed the taking of the 
“chart” plates for the Astrographic Survey. 
CAMBRIDGE OBSERVATORY.—Embodied in the Cambridge 
University Reporter for June 16, is the annual report of the 
Cambridge Observatory from 1898 May 26 to 1899 May 25. 
With the meridian circle, 2241 observations of 1420 stars have 
been taken, most of these being repetitions of previous observy- 
ations for the catalogue. 
One hundred and seventy-six observations have been made of 
the Harrow occultation stars ; and, at the request of Dr. Gill, 
observations of heliometer comparison stars were commenced 
in March and are still in progress. 
In addition, there have been other measurements of standard 
stars, bringing up the total number of meridian observations to 
3016. 
The new bent equatorial, to be called the ‘‘ Sheepshanks equa- 
torial” (see illustrated description in A/onthly Notices, R.A.S., 
1899 January, vol. lix. p. 152) was completed about September 
1898, and its adjustment was undertaken by Mr. Hinks. It was 
soon found that the objective tube had a large flexure, and a new 
tube is being made, The first trial photographs were unsatis- 
factory, the disturbing cause being thought to be the air currents 
in the tube, which is partly open near the joint. 
The Newall telescope has been employed on 96 nights 
during the year, in connection with the Bruce spectroscope, in 
taking photographs of stellar spectra for determining their 
velocity in the line of sight; 150 photographs have been ob- 
tained, giving material for determining the velocity of 60 stars. 
Thirty of these are included in the Potsdam list of 51 stars 
observed from 1888-1891. Preparations are in hand for con- 
verting the spectroscope into a powerful 4-prism instrument 
for detailed examination of a few of the brightest stars. 
Special series of stellar spectra have been taken to assist in 
the reduction of the eclipse photographs obtained in India 
in 1898. For this purpose also attempts have been made to 
separate scandium salts from the mineral gadolinite. 
PICTURES PRODUCED ON PHOTOGRAPHIC 
PLATES IN THE DARK: 
if THINK I may fairly assume that every one in this theatre 
has had their photograph taken, and consequently must 
have some idea of the nature of the process employed. TI have, 
therefore, only to add, with regard to what is not visible in the 
process of taking the picture, that the photographic plate is a 
piece of glass or such like body, coated on one side by an 
adhesive paste which is acted on by light, and acted on in a 
very remarkable manner. No visible change is produced, and 
the picture might remain latent for years, but place this acted 
on plate in a solution, of, say pyrogallol, and the picture 
appears. The subsequent treatment of the plate with sodium 
hyposulphite is for another purpose, simply to prevent the con- 
tinuance of the action when the plate is brought into the light. 
Now, what I purpose demonstrating to you to-night is that 
there are other ways of producing pictures on photographic 
plates than by acting on them by light, and that by these other 
means a latent picture is formed, which is rendered visible in 
precisely the same way as the light pictures are. 
The substances which produce on a photographic plate these 
results, so strongly resembling those produced by light, are, 
some of them, metallic, while others are of vegetable origin. 
At first it seemed very remarkable that bodies so different in 
character should act in the same way on the photographic plate. 
The following metals—magnesium, cadmium, zinc, nickel, 
aluminium, lead, bismuth, tin, cobalt, antimony—are all capable 
of acting on a photographic plate. Magnesium most strongly, 
antimony but feebly, and other metals can also act in the same 
way, but only to a very slight extent. The action in general is 
much slower than that of light, but under favourable conditions 
a picture may be produced in two or three seconds. 
1A lecture delivered at the Royal Institution on Friday, May 5, by Dr. 
W. J. Russell, V.P.R.S, 
NO. 1548, VOL. 60] 
Zinc is nearly as active as magnesium or cadmium, and 
is the most convenient metal to experiment with. In its 
ordinary dull state it is entirely without the power of acting 
on a photographic plate, but scratch it or scrape it, and it 
is easy to prove that the bright metal is active. I would say 
that all the pictures which I have to show you, by means of 
the lantern, are produced by the direct action of the metal, or 
whatever the active body may be, on the photographic plate, 
and that they have not been intensified or touched up in any 
way. This first slide is the picture given by a piece of ordinary 
zinc which has been rubbed with some coarse sand paper, and 
you see the picture of every scratch. Here is a piece of dull 
zinc on which some circles have been turned. It was exposed 
to the photographic plate for four hours at a temperature of 
55 C. In the other cases, which are on a larger scale, a zine 
stencil was polished and laid upon a photographie plate, and 
you see where the zine was in contact with the plate much 
action has occurred. In the other case a bright zine plate was 
used, and a Japanese stencil interposed between it and the 
photographic plate, and a very strong and sharp picture is the 
result. The time required to produce these zinc pictures varies 
very much with the temperature. At ordinary temperature the 
exposure would have to be for about two days, but if the tem- 
perature was, say, 55° C., then half to three-quarters of an hour 
might be sufficient. Temperatures higher than this cannot be 
used except for very short times, as the photographic plate 
would be damaged. Contact between the zinc and photographic 
plate is not necessary, as the action readily takes place through 
considerable distances. Obviously, however, as you increase 
the distance between object and plate, so you decrease the 
sharpness of the picture, as is shown by the following pictures, 
which were taken respectively at a distance of 1 mm. and 3mm. 
from the scratched zine surface. The appearance of the sur- 
faces of different metals varies, and the following slides show 
the surface of a plate of bismuth, a plate of lead, and one of 
aluminium. On the next slide are the pictures produced by 
similar pieces of pure nickel and cobalt, and it clearly shows 
how much more active in this way nickel is than cobalt. Many 
alloys, such as pewter, fusible metal, brass, &c., are active bodies, 
and in the case of brass the amount of action which occurs is 
determined by the amount of zinc present. Thus you will see 
that a brass with 30 per cent. of zinc produces hardly any action 
on the photographic plate, but when 50 per cent. of zinc is 
present there is a fairly dark picture, and when as much as 70 
per cent. is present a still darker picture is produced. The 
second class of bodies which act in the same way on a photo- 
graphic plate are organic substances, and belong essentially to 
the groups of bodies known as terpenes. In trying to stop the 
action of metallic zinc, which I thought at the time might arise 
from vapour given off by the metal, copal varnish was used, but in 
place of stopping the action it was found to increase it, and this 
increase of activity was traced to the turpentine contained in the 
varnish. In experimenting with liquids it is convenient to use 
small shallow circular glass vessels such as are made for bac- 
teriological experiments, the plate resting on the top of the 
vessel, and the amount of liquid in the vessel determining the 
distance through which the action shall take place. The fol- 
lowing slide, produced in this way, shows how dark a picture 
ordinary turpentine produces. All the terpenes are active 
bodies. Dipentine is remarkably so; in a very short time it 
gives a black picture, and if the action be continued, the dark 
picture passes away, and you then have a phenomenon corre- 
sponding to what photographers call reversal. The strong 
smelling bodies known as essential oils, such as oil of bergamot, 
oil of lavender, oil of peppermint, oil of lemons, &c., are all 
active bodies, and all are known to contain in varying quantities 
different terpenes ; therefore ordinary scents are active bodies, 
and this is shown by the following pictures produced by eau 
de Cologne, by cinnamon, by coffee, and by tea. Certain 
wines also act in the same way, Sauterne gives a tolerably dark 
picture, but brandy only a faint one. Other oils than these 
essential ones are also active bodies ; linseed oil is especially 
so ; olive oil is active, but not nearly as much so as linseed oil ; 
and mineral oils, such as paraffin oil, are without action on the 
photographic plate. 
Interesting results are obtained with bodies which contain 
some of these active substances ; for instance, wood will give 
its own picture, as is shown by the following slides : the first is 
a section of a young spruce tree, the next a piece of ordinary 
deal, and the third of an old piece of mahogany. Again, the 
