35 
with a photographic light sensitive film, which latter has to be 
colour sensitive to all colours, and when exposed through the 
layer of transparent colours will be exposed, or acted on, in 
just the proportions of the different lights passed. 
When such an exposed plate is developed it will be a 
negative, as in all such photographic processes, and to show the 
plate in its true colour effects a positive picture is required. It is 
therefore necessary to reverse the photographic light action on the 
plate, so that opacity is formed where transparency is shown in 
the negative, or where the light has not acted on, and vice versa. 
There is one great drawback to the processes belonging to this 
group, that is, duplicates will never show the same brilliancy as the 
first transparency made, if such duplicatescan be made. A further 
drawback is, that colour printing from such transparency is for 
the present out of the question, and all advances in such directions 
have to be accepted with a fair amount of reserve for a good 
while to come. 
Group 4.—The subtractive method of photography in 
colours, also foreshadowed by Ducos du Hauron in 1869, is 
practically the only method allowing the colour printing to be 
done by the aid of three negative colour records, giving thereby 
facilities for multiplication in nearly any colour printing process. 
One Negative is required which gives all the yellow as trans- 
parent, all the greens and oranges, more or less transparent, that 
is just in such proportions as yellow is present ; such a negative 
colour record is obtained by any ordinary dry plate as sold in the 
market. Ora like negative colour-record can be obtained by a 
colour sensitive plate which receives all light through a blue violet 
colour filter, a filter filtering out, cutting-out, making non- 
acting all colours but blue. 
A second Negative is required which has to be printed in red, 
and for that purpose the red colour of any object to be pkoto- 
graphed has to be rendered by transparency in the negative-record 
and the blue must be shown by half-transparency. Such a 
negative we obtain with an ortho-chromatic, that is erythrosin 
bathed plate and an adjusted yellow filter, or with a plate 
sensitised to all colours when exposed to light, the latter has 
to pass or is filtered through a green filter, which latter cuts out 
or makes non-acting the light rays which are not required to act 
on the photographic plate. 
A third Negative, which represents the exact opposite as the 
first named negative and has to be printed in blue, shows there- 
fore this colour by transparency and is obtained through an 
orange red filter on a photographic plate which is sensitive to all 
colours. This filter, if properly adjusted, will not allow any blue 
active rays to pass. The absence of light action on the sensitive 
plate will here, as in the other negatives, indicate the colour in 
which to print. 
