Vol. XXV. No. 10.] 
POPULAE SCIENCE NEWS. 
153 
Mount Veniou. To Mount Veiuon, also, were the 
nienibers of the A. A. A: S. taken as a body, be- 
sides being received by the Boar<i of Trade at the 
rooms of Arlington Hotel. The National Museum 
was opened for them one evening, as also to the 
International Geologists a week later. For the 
International Geologists there were many recep- 
tions, the most notable being that of the Geologi- 
cal Society at the Arlington Hotel, and of the 
Director and officers of the Geological Survey in 
the rooms appropriated for their work. 
Many indeed were the demands made by science 
upon Washington this August, but Washington 
was ready to meet them all, and every scientist or 
science lover who found himself there must ac- 
knowledge that his pilgrimage was not in vain. 
PHOTOGRAPHY WITH A BLACK BACK- 
GROUND. 
The color known as black is not, properly 
speaking, a color at all, but simply the absence of 
all color or light. A substance that absorbs all 
the rays of light falling upon it would be per- 
fectly black: but no such substance is actually 
known, as all dyes or pigments, even of the dark- 
est shades, reflect a small quantity of the light 
falling upon them. 
As bl.ack, however, is the total absence of light, 
it follows that nothing can be so black as a per- 
fectly dark room, or the out-door world on a 
cloudy night ; and, as under such couditions there 
Fig. 1. 
can be no light, there will be no action upon a 
photographic plate exposed in such a room or 
upon such a night. Thei'efore, if we desire to 
take a photogi-aph against a perfectly black back- 
ground, we need not trouble ourselves w ith hang- 
ings of black cloth, but simply open a window on 
a cloudy evening, if we are to use a flash light, or, 
if the picture is to be taken by daylight, we may 
open the door of a dark room or building and 
obtain a background both theoretically and prac- 
tically non-actinic. 
With such a non-actinic background many in- 
teresting and amusing photographs may be made 
by a process of multiple exposure. J^ig. 1, for 
instance, represents a man. playing ball with his 
own head. This picture was taken at night by 
tneans of a flash light, three exposures being 
necessary, the open window forming the back- 
ground. The two small heads were first taken in 
succession, the person represented standing on 
the balcony outside, with a black cloth thrown 
over his body, and the light from a flash lamp 
being thrown upon his head as he posed in the 
difterent positions. He then took the position 
represented in the engraving, and another flash 
imprinted his whole figure and the walls of the 
room upon the plate, which, when developed, gave 
the amusing result shown in the illustration. 
Fig. «. 
Fig. 2 was taken in a similar manner, the statu- 
ette which apparentlj- stands upon the table being 
a miniature of the girl kneeling at its side. 
In Fig. 3 the conditions are dHferent. This pic- 
ture was taken by daylight, and the open door of 
a warehouse formed the non-actinic background. 
Here the secondary image is enlarged, and the 
small boy in the shafts of the wagon is appar- 
ently engaged in carrying off his own head. 
Numerous other applications of this principle 
w ill at once occur to every photographer, and the 
process is so simple that very unique eftects can 
be ol)tained by any amateur possessed of the ordi- 
nary amount of patience and ingenuity. 
rig. 3. 
An important point in such pictures is to have 
the difterent images fall upon their proper place 
on the plate. This is easily secured by attaching 
to_the ground glass a bit of gummed paper show- 
ing the position of the smaller images — the beads 
in Fig. 1, for example. Then when the principal 
exposure is made, the relative position of the dif- 
ferent images can be accurately adjusted by mov- 
ing the camera in the usual way. 
The accompanying engravings are copied from 
La Nature. 
*»< 
[Special CorreBpoudence of Popular Science News.] 
PARIS LETTER. 
The university holidays have begun, and sci- 
ence enjoys well-deserved vacations; no lectures, 
no laboratory work ; scientific societies — even the 
Academie des Sciences — are nearly abandoned, and 
writers have for a month or two dropped their 
pens, while the ink slowly dries in the inkstand. 
But this does not prevent some work from being 
done, and most of the scientific men devote a 
week or a fortnight to scientific meetings, which 
commonly occur at this time of th^ year. Some 
— many even — have been to the meeting of Hy- 
giene and Demography in London ; others to the 
Cardifl' meeting of the British Association for the 
Advancement of Science; many have followed 
the sittings of the Congres pour I'Etude de la Tu- 
berculose in Paris ; many, also, are going to the 
Marseilles meeting of the Association pour VAvance- 
ment des Sciences. All these meetings are of inter- 
est and pleasurable. In the French meeting for 
the study of tuberculosis important discussions 
have been conducted. Of Koch's method nothing 
has been said ; it is totally abandoned, and is con- 
sidered a total failure. The method which has 
been most discussed is that of MM. C. Richet and 
Hericourt, of Paris, which consists of subcuta- 
neous injections of dog"s blood serum, or hemo- 
cyne, as it is called. This method has been exper- 
imented with in France and in Italy by many 
physicians, and, on the whole, seems to yield 
good results. The principal effect obtained is the 
considerable benefit conferred upon the digestive 
functions. Patients who were unable to eat any- 
thing and were utterly dyspeptic have seen their 
appetites wonderfully improved, and were aston- 
ished at the quantity of food they could absorb. 
This improvement is accompanied by an increase 
in weight, at times very important; and other 
results consist in the greater ease of breath, in 
quieter sleep, in a marked decrease of cough and 
of night transpiration, so that the patient feels 
really stronger and is less troubled with his dis- 
ease. At the same time it is noticed that the 
bacilli disappear, if the patient is at the first 
period of the disease, while they become much 
less frequent if the second period has set in. 
Patients at the third and last stage are sometimes 
benefitted, but, of course, their case is much less 
hopeful than that of the others. Such are the 
general results noted by MM. Richet and Heri- 
court, and by Professor Semmola, of Naples, M. 
Vidal, of Hyeres, and many others; and, as the 
experiments have been conducted for a period of 
many months already, if not a whole year, these 
conclusions may be considered as based on a suf- 
ficient number of data. Some other facts have 
been gathered which deserve a passing notice. 
Professor .Semmola has seen that the capacity of 
the chest, or lungs, is increased by hemocyne, and 
that the blood contains more hemoglobin than 
before, so that the vitality and efficiency of the 
blood is much increased. Tubercular diseases of 
most organs are improved as well as lung tuber- 
culosis, and, in fact, in all cases of tuberculosis, 
wherever it may be seated, a marked progress 
takes place. The fact that hemocyne is conducive 
to an increase in appetite, weight, and general 
health has prompted Professor Pinard, the well- 
known obstetrician, to use it for new-born chil- 
