33 
starch grain filter. Exposure is made, the plate developed, and 
then, before the plate is fixed, the negative image is dissolved 
away and the remainder of the sensitive emulsion film re-developed, 
thereby producing a positive corresponding to the negative first 
developed. The plate is then fixed, washed and dried, and 
varnished. Let us trace what has happened in this process in 
the light of the three primary colours of white light, viz., red, 
green and violet. We will suppose that we are photographing an 
object that is pure red, #e, the colour of the red grain in the 
filter of the plate. The rays from this object being red only, 
would be obstructed by the green and violet grains in the filter 
and would produce no effect on the sensitive plate, but they 
would pass through the red grains and would affect the emulsion, 
producing a deposit of silver when the plate was developed. 
On the plate at this stage being illuminated from behind by 
white light, this deposit (the first negative image) would obstruct 
the light coming from immediately behind it, that is to say, it 
would cut out the light coming through the red parts of the filter, 
but the other portions of the filter having no obstruction on them, 
due to the silver image, would pass their respective colours, viz., 
green and violet, with the result that we should have the red 
object represented in the colour which is the result of mixing 
lights of these two colours, that is to say greenish-blue. In other 
words, a colour complementary of the red is produced. But when 
the negative image has been converted into a positive one, the 
conditions are precisely opposite as regards the obstruction offered 
to the light by the silver deposit. Now the green and violet 
grains and the red is bared ; with the result that on the picture 
being examined by light passing through it, the light reaches the 
eye only through the red portions of the filter film and the object 
is seen in its actual colour. 
I am indebted to Mr. George Brown, Editor of the “ British 
Journal of Photography,” for many of the very fine specimens of 
landscape and flowers shown. Also to Mr. Stanley Read, 
Secretary of the Hove Camera Club, who has kindly lent me his 
collection. The microphotographs are prepared by Mr. Caush, 
and are the first I have seen of the kind; I am sure no better 
ones could be produced. 
