Nov. 2, 1882} 
NATURE 
15 
about a fortnight, and have been up the Binué as far as 
Loko, about 100 miles, where I got some birds. Alto- 
gether up to the present I have seen or got about 80 
species of birds, including Scopus, Plotus, Indicator, and 
Rynchops ; as yet no Podica, Irrisor, or Musephagide. 
Of Hornbill I have seen 3 or 4 species, but they are very 
shy, and as yet I have not shot one. Ploseine birds are 
the feature here; about 1-3rd of the species are of that 
family, and some I have are good ones, especially stre/da 
nigricollis and E. rara, both of them discovered by 
Heuglin. These and other things make me fancy that 
we are out of the true West African region here; the 
antelopes seem also eastern. There are 4—5 here, in- 
cluding a brown Aippotragus, and what I fancy is Aéce- 
laphus tora. \ have skins and horns of these, and shall 
get others. Bos brachyceros is common here, but as yet 
I have only seen spoor, not the beast itself. We saw lots 
of Hippopotamuses coming up, and I killed the second I 
shot at, but could not recover the body. 
I have also killed a large crocodile, 15 feet long, ap- 
parently C. acutus. I have also a few fishes and reptiles, 
and shall get more I hope. Butterflies are not very 
numerous at present, and the country is too open for 
them, being, generally speaking, a large grassy plain, 
with lots of isolated trees, not very big, and bushes. There 
is no regular thick forest up here at all, and even in the 
lower river, in the delta, it is nothing like the Neotropical 
forests. The weather has been very dry, and the river is 
still rising. After leaving Bidda our plans are uncertain. 
Mr. M. talks of going on to Sokoto, if he can get away 
from his stock-taking, and if he goes I shali probably go 
too. If not, I shall try and stay some time at Ischunga, 
a station a little off the river above Egga.~ 
We are happy to be able to add that Mr. Forbes was 
in excellent health at the date of his letter. 
THE INFRA-RED OF THE 
SPECTRUM 
T is with a certain amount of dread of boring the 
readers of NATURE, that I have taken up my pen 
to write on the method of photographing with rays 
of very low refrangibility, since it ought to have passed 
the limits of novelty. And yet I suppose it has not alto- 
gether done so, since almost weekly, I have inquiries 
made as to where the method is described, and am 
questioned as to how to succeed with it, when my corre- 
spondents know where to find its description. The 
Editor, also, has asked me to write on the subject, so I 
propose to put as concisely as I can what plan to adopt. 
It is almost too well worn a scientific adage to repeat that 
unless you can obtain a sensitive salt which will absorb 
the rays to be used photographically, you cannot hope 
for success; and the method which I shall describe pre- 
sently fully secures this desideratum. To photograph the 
red and dark rays then a sensitive salt must be procured 
which shall absorb the red and ultra-red rays. The colour 
of the salt to aim at then is a bluish green, which gives a 
continuous absorption at the least refrangible end of the 
spectrum. The salt employed is bromide of silver ina 
modified molecular state, a state I may say which is very 
easy to obtain when the formula below is strictly carried 
out, but very easily missed if the experimenter is self- 
inspired to make improvements in the method of pro- 
cedure. I don’t know whether it is something peculiar to 
photographic minds that there is in them such a large 
amount of self-assurance, but my frequent experience is 
that those who try a formula for a photographic prepara- 
tion invariabiy try to improve on it before giving the 
original one a chance of success: and then when failure 
occurs they blame everything and everybody except their 
own conceptions. May I ask those who read this and 
endeavour to prepare the sensitive compound alluded to, 
WORK IN 
to follow out strictly the directions as I described them 
in the Bakerian Lecture for 1880. 
The following is the mode of preparation. A normal 
collodion is first made according to the formula below :— 
Pyroxyline (any ordinary kind) 16 grains 
IDRIS YAS Sey)! a a la) ig ae 4 02. 
Alcoholl(G820)hiemwernen fey 2 OZ. 
This is mixed some days before it is required for use, and 
any undissolved particles are allowed to settle, and the 
top portion is decanted off. 320 grains of pure zinc 
bromide are dissolved in } 07. to 1 oz. of alcohol (*820) 
together with 1 drachm of nitric acid. This is added to 
3, ozs of the above normal collodion, which is subsequently 
filtered. 500 grains of silver nitrate are next dissolved in 
the smallest quantity of hot distilled water, and 1 oz. of 
boiling alcohol ‘$20 added. This solution is gradually 
poured into the bromized collodion, stirring briskly while 
the addition is being made. Silver bromide is now 
partially suspended in a fine state of division in the collo- 
dion, and if a drop of the fluid be examined by transmitted 
light it will be found to be of an orange colour. ; 
Besides the suspended silver bromide, the collodion 
contains zinc nitrate, a little silver nitrate, and nitric acid, 
and these have to be eliminated. The collodion emulsion 
is turned out into a glass flask, and the solvents carefully 
distilled over with the aid of a water bath, stopping the 
operation when the whole solids deposit at the bottom of 
the flask. Any liquid remaining is carefully drained off, 
and tke flask filled with distilled water. After remaining 
a quarter-of-an-hour the contents of the fla k are poured 
into a well-washed linen bag, and the solids squeezed as 
dry as possible. The bag with the solids is again im- 
mersed in water, all lumps being crushed previously, and 
after half-an-hour the squeezing is repeated. ‘Chis opera- 
tion is continued till the wash water contains no trace of 
acid when tested by litmus paper. The squeezed solids 
are then immersed in alcohol ‘820 for half-an-hour to 
eliminate almost every trace of water, when after wringing 
out as much of the alcohol as possible the contents of the 
bag are transferred to a bottle, and 2 ozs. of ether (720) 
and 2 ozs. of alcohol (*805) are added. This dissolves the 
pyroxyline and leaves an emulsion of silver bromide, 
which when viewed in a film is essentially green-blue 
by transmitted light. 
All these operations must be conducted in very weak 
red light—such a light, for instance, as is thrown by a 
candle shaded by ruby glass, at a distance of twenty feet. 
If a green light of the refrangibility of about half way 
between E and D could be obtained it would be better 
than the faint red light transmitted by ruby glass, since 
the bromide is less sensitive to it than to the latter. The 
light coming through green glass after being filtered 
through stained red glass is almost the best light to use. 
It is most important that the final washing should be 
conducted almost in darkness. It is also essential to 
eliminate all traces of nitric acid, as it retards the action 
of light on the bromide, and may destroy it if present in 
any appreciable quantities. To prepare the plate with 
this silver bromide emulsion all that is necessary 1s to 
pour it over a clean glass plate, as in ordinary photo- 
graphic processes, and to allow it to dry in a dark 
cupboard. ; 
It has been found advantageous to coat the plate in yed 
light, and then to wash the plate and immerse it ina 
dilute solution of HCl, and again wash, and finally dry. 
These last operations can be done in dishes in absolute 
darkness ; the hydrochloric acid renders innocuous any 
silver sub-bromide which may have been formed by the 
action of the red light, and which would otherwise cause 
a heated image. : 
Let me here give warning, that the emulsion formed 
will be very grainy in appearance, and requires vigorous 
shaking to cause it to emulsify proper. If it requires a 
little plain pyroxyline, say about two grains to the 
