16 
NATURE 
[Voo. 2, 1882 
fluid ounce should be added to give greater consist- 
ency. One thing is certain, if it be not coarse-grained 
under the microscope it will not be sensitive to the re- 
quired region, and moreover it will be found that on an 
average it should be about twice as coarse as the average 
form of bromide which is generally obtained in collodion 
emulsion. Here let me interpolate a remark. It has 
been assumed that because an emulsion in gelatine has a 
bluish colour after it has been boiled, that in this case we 
have the same form of bromide as that described above. 
It is a very different form : let me show how. Suppose 
we throw a spectrum on a gelatine plate it will be found 
that G requires about + of a second with a very narrow 
slit, whereas to obtain B it will require the best part of a 
minute, and to obtain rays of lower refrangibility very 
much more ; and that any amount of exposure will not 
make an impression much below A. With the blue-green 
bromide in collodion to obtain an impression about G will 
take some eight or ten seconds, and it will be found that 
at the same time we have an impression of B. A minute’s 
exposure to the prismatic spectrum will under similar 
circumstances give an impression as much below A as D 
is above it, measured not in wave-lengths but along the 
photograph. I point out this because a leading conti- 
nental photographic experimentalist has expressed himself 
satisfied as to the identity of the two forms of sensitive 
salt. They are totally distinct as if he tried to work 
with a gelatine plate in the infra-red region he will 
soon own. Now in reference to the coarseness of 
grain it is right to call attention to its disadvantages. Its 
advantage we know. In spectrum work we often come 
across close pairs of lines. Now suppose each pair 
happened not to be separated by a larger interval than 
the grain of the sensitive salt, we shall be unable to 
resolve such a pair, for the action of either component of 
the pair, and much more both, if they fell on it would be 
to cause, on development, a reduction to metallic silver of 
the whole grain. Thus evidently such a close pair would 
be unresolved. 
When a photograph of the spectrum on the finest 
grained plate is examined under the microscope it will be 
found that the metallic image is composed of grains of 
silver and nothing else; and that instead of the lines 
having sharp edges as seen by the eye that they shade. 
Part of this shading is due to the grain, though the greater 
part is due to proper absorption, which the eye is incap- 
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able of distinguishing. The fineness of grain given by 
the different processes we may class as follows, in the 
order of coarseness, the coarsest grain being first :— 
1. Wet plate developed by iron. 
2. Special bromide emulsion, as before described. 
§ Ordinary collodion emulsion. 
3 1 Wet plate developed by pyrogalic acid. 
4. Gelatino-bromide plates. 
It will thus be seen that for delicate work the dispersion | 
with the wet plate process and the special bromide emul- 
sion must be larger than when using a gelatine plate if 
equal resolving power be wished for. The above plate 
is an instance of this. In it we have the solar spectrum 
in approximate wave-lengths from Ad (7,600) to about | 
A 10,500. The general impression to the eye is the 
extraordinary width of the lines compared with those in 
the visible spectrum. No doubt they are as a rule 
broader, but their breadth is also to be accounted for in 
other ways. 
Secondly, the dispersion used was the first order of a 
Rutherford grating 17,200 lines (about) to the inch, and a 
camera lens of a focus of about fifteen inches. In later pho- | 
tographs nearly all the broad lines have been resolved into 
pairs or triplets,as have also some of the lines of medium 
breadth. There are lines, however, like the 3 broad lines 
between 8500 and 8700 which remain unchanged whatever 
dispersion was used. This resolution was effected by using 
| a finer slit and dispersion of the second order, the fine slit 
| 
| 
| 
| 
alone will not giveit. If we take an example in the visible 
spectrum, and examine the B line with the eye. it will be 
found to be made up of a series of doublet flutings, each 
component being apparently of equal intensity. These 
pairs it is impossible to secure on the photographic plate, 
unless the second order of the grating spectrum is used ; 
but when secured it will be found that the more refrangible 
component is more intense, as is the case in certain 
hydro-carbon flutings. The sole reason why the first 
order is useless to cause resolution is that the pairs are so 
close they can both fall on the diameter of the grain of the 
sensitive compound. On the other hand, with a gelatine 
| plate 1 have been able to see on one inch and a half every 
| line and more than given in Angstrém’s map from G to 
First, the slit used was not quite as fine as | 
might have been when the photographs were taken. | 
F. In this case the grain is almost invisible. 
The development of the plate is greatly more difficult 
than the preparation of the emulsion. A strong developer 
it will not stand, and I may say also that a very new one 
is also inadmissible when using the ferrous oxalate de- 
velopment. To make the developer a saturated solution 
of neutral oxalate of potash is saturated in the cold, with 
