656 
CAPTAIN W. DE W. ABNEY ON THE PHOTOGRAPHIC METHOD OF 
film by transmitted light appeared of a sky-blue colour, inclining to a green tint, 
visibly absorbing the red. In this condition it was sensitive to the whole spectrum, 
visible and invisible. There was much uncertainty attaching to the preparation ; 
about one batch of the salt suspended in collodion out of five or six fulfilling the 
requisite conditions. I am now, however, in a position to prepare it without any 
risk of failure, my experiments of the last nine months having showed the conditions 
absolutely necessary for success. The following is the mode of preparation. A normal 
collodion is first made according to the formula below :— 
Pyroxyline (any ordinary kind).16 grains 
Ether ('725 Sp.) ... 4 oz. 
Alcohol ('820). 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 l oz. 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 '820 added. This solution is gradually poured into 
the bromized collodion, stirring briskly whilst the addition is being made. Silver 
bromide is now suspended in a fine state of division in the collodion, 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 the flask filled with 
distilled water. After remaining a quarter-of-an-hour the contents of the flask are 
poured into a well-washed linen bag, and the solids squeezed as dry as possible. The 
bag with the solids is again immersed in water, all lumps being crushed previously, 
and after half-an-hour the squeezing is repeated. This operation 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 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 20 feet. It 
* 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. 
