Joty—On a Method of Photography in Natural Colours. 129 
manner, it is requisite here to describe, briefly, Maxwell’s method of composite 
colour photography as developed by Mr. Ives. 
Composite colour photography deals with the subjective reproduction of all 
visible wave-lengths in two stages; a photographic analysis and an optical 
synthesis. In the first operation the several wave-lengths are caused to produce 
three separate photographic images according to their physiological activity in 
exciting the supposed fundamental red, green, and violet sensations, Suppose, 
as a simple example, that we are photographing the yellow of the spectrum near 
the D line. One of the plates must record an image of the spectrum at this 
point having a density of silver deposit corresponding to the degree in which 
this wave-length can excite the red-seeing nerve, and a second must acquire a 
density corresponding to the degree in which this same wave-length can excite 
the green-seeing nerve. The third plate records no impression, for the wave- 
lengths near D excite no violet sensation; but this yellow sensation is the 
resultant of two physiological effects only, a red and a green sensation in certain 
proportions. The nature of these proportions can be ascertained by colour 
measurements effected upon colour sight. We have now obtained three 
negatives possessing densities of silver deposit corresponding to the degrees 
in which the three several fundamental colour sensations are excited. These 
degrees of density will be interpreted as degrees of transparency in the 
positives. The first positive, if backed with a red glass, will transmit a 
quantity of red light corresponding to the intensity of the physiological excita- 
tion of redness in the ‘red’ nerves; the second, backed with green, similarly 
represents the stimulation of the ‘green’ nerves by the yellow colour of the 
object ; the third positive is backed with blue-violet glass, but is quite opaque, 
and no violet light is transmitted through it. The projection now of all three 
images superposed upon the screen forms the second stage of the procedure; the 
optical synthesis of the original colours. The eye, regarding the superposed 
image, receives in fact the same amounts of red and green sensation, and 
experiences the same absence of violet sensation which would have attended the 
formation of the image of this part of the spectrum upon the retina. 
This process, if accurate reproduction of colour is sought, necessitates the 
use of two distinct sets of colour selective screens; for the analysing screens will 
by no means possess the tints ultimately required in the optical synthesis. This 
will be evident when it is considered that the wave-lengths which most strongly 
stimulate the several fundamental sensations are not those which most nearly 
represent those sensations to the normal eye. The C red, for example, is not the 
wave-length which most strongly stimulates the red sensation; a wave-length 
which appears orange to the eye possessed of both red and green vision, will far 
more efiectively excite the red sensation. Hence, in order to photograph the 
X 2 
