Nov. 2, 1882 | 
NALORE 
17 
ferrous oxalate : and then the deep red solution decanted 
off. When freshly prepared it is useless to attempt to 
develop a plate with it unless the precaution be taken 
of adding to it an equal part of a saturated solution of 
ferric oxalate in the oxalate of potash. Such a mixture 
may be employed by adding to it immediately before all 
an equal volume of a solution of potassium bromide 
(twenty grains of the salt to thirteen of water). The 
plate may then develop without fog or it may not : if it 
does fog, the developer must have more bromide solution 
added to it, and another trial made. On some daysa 
clean picture seems ar impossibility, whilst on others, 
every one will be perfect. It is not the emulsion that is 
in fault, since, on a ‘‘clear day” and ona “foggy day ”’ 
the identical emulsion may be used, showing that the 
developer is at fault. This year this trouble seems to 
have increased, and I can only lay it down to the different 
preparations of the oxalates. Of one thing care should be 
taken, viz., that the developer never shows alkalinity ; a 
drop cf dilute sulphuric acid or nitric acid may be added 
to the developing cup just before development with | 
advantage. 
With prisms the photography with the rays of low | 
refrangibility is simple, with one great drawback, and that | 
is the difficulty of obtaining a true focus for the plate. 
This must all be done by guess-work, and plates exposed 
till the focus is obtained. When once obtained it is a 
good plan to mark the camera to show the focus, and at 
“the same time accurately to mark the table on which it 
stands, so that the same portion of the lens recetves the 
same rays. This is more particularly necessary to attend 
‘to when using an achromatic lens. I believe it to be 
easier to use the uncorrected lens than a corrected one, 
provided always that the camera has a propery horizontal 
swing back, which can be shifted through a very large 
angle at least 30° when using three prisms. If a spherical 
mirror be used in the collimator and in the camera instead 
of the lenses, the same difficulties of focusing do not pre- 
sent themselves. The disadvantage of this method is that 
the edges of the spectrum based are diffused and not 
straight, and this is awkward when making comparison 
of different spectra. With a grating nearly the same 
difficulty arises when using lenses, but not quite to sucha 
degree. If “a” and A be got in focus at the end of the 
plate, the swing back being used till this results, and if 
the lens be placed close to the grating, the whole of the 
infra-red region will be fairly in focus. This of course 
only applies to my own grating which may havea slight 
curvature. In using the grating we must not forget that the 
second order overlaps the first order, and the third order 
the second order and so on: and if a plate were exposed 
without any artifice being adopted to get rid of this over- 
lap the plate would show two or three spectra. There 
are several methods of accomplishing this separation, the 
simplest being to use the absorbing medium in front of 
the slit. At first I used stained red. glass which cuts off 
all radiation above the green, leaving thus the tails of the 
different spectra intact. ‘At present, when wishing to go no 
further down the scale than A 30,000, I have found that a 
deep coloured solution of iodine of potassium in water about 
one-tenth of an inch in thickness is very excellent. The 
objection to the red glass is that it exercises a certain 
amount of general absorption in the infra-red region, 
but with the “white glass of the cell holding the solu ion, 
and the solution itself, this general absorption is mini- 
mized. To get down still further, very thin stratum of 
a blue dye in tetrachloride of carbon is efficacious in 
conjunction with the iodine solution. With the above 
solutions \ 13,000 can be reached. Beyond this limit it is 
necessary to. use other means of eliminating the higher 
orders of the spectrum. The simplest plan is to place 
behind the collimator a couple of prisms, and some 
two feet from the prisms, the grating so that it only 
receives those rays which it_ may be desired to 
im- 
press. Thus one side of the grating may catch the 
limit of the red whilst the rest will be filled with the dark 
rays. The most difficult planis to place a prism according 
(as Frauenhofer did) in front of the grating, in such a 
way that the axis of the prism is at right angles to the 
ruling and parallel to the plane of the grating ; this causes 
a complete separation of all the different orders of spectra. 
But the resulting photographs are inconvenient to 
measure, since they are curved, and the position of the 
camera is also ewkward. Another plan is to use a prism 
in front of the slit, but this too, I have found incorvenient 
for the same reasons as given above. For ordinary work 
the absorption method is decidedly the most elegant, but 
then it limits the operations with the spectrum. It was 
from photographs obtained in this manner that the above 
map of the solar spectrum was obtained, and as it is before 
us, it may be well to make afew remarks onit. As to 
CARBON: DISULRAIDE- 
00a 
CHLOROFORM 
= ee | ime ne 
METHYL» 10D1DE 
Om NOON UNTNRC2 
ETHYL GID DE 
- BENZING 
a 
AMYLENE 
what the lines are due to we are at present absolutely unin- 
formed, except as to some very few. A notable exception 
to this is the line lettered about 8600, which is one of the 
strongest lines in this part of the spectrum. Colonel 
Festing and myself found that this line coincided abso- 
lutely in position with what we call the radical absorption 
band of benzene, that is to say, that by diminishing a 
thin layer of benzene placed between the slit of the spec- 
troscope and a source of light giving a continuous spec- 
| trum, this absorption-band, amongst many others, was 
the last to disappear, and that it also was the key-note as 
it were of the absorptions of all benzene derivatives. 
A coincidence of this kind would not be fortuitous any 
more than that the vapour of sodium gives lines coin- 
cident with the D lines ; and hence we were forced to 
ascribe this line to benzene or some of its derivations. 
When first we made this announcement it was facetiously 
