128 Joty—On a Method of Photography in Natural Colours. 
photography is specified. While this definition of the theory is clear, the wording 
of the report of his lecture leaves some doubt as to the actual nature of the light 
filters used by Maxwell. There is evident confusion in the wording of the 
report, as appears in the reference to the “three photographs introduced into the 
camera.” 
I cannot but think, however, that the three solutions referred to are those used 
by Maxwell to transmit the three primary colour sensations. This view is 
strengthened by his previous statement that the red sensation is stimulated most 
by the red wave-lengths, but also by the orange and yellow wave-lengths, &e. 
This mistake would quite explain his use of the solutions transmitting the primary 
colours. 
It is further of interest to note that at a yet earlier date Maxwell describes 
his idea of using our knowledge of the colour sensations for obtaining photographs 
in natural colours. As early as 1855, in a paper read before the Royal Society 
of Edinburgh, ‘‘On Experiments on Colour as perceived by the Eye,”* he 
describes how by triple projection of three photographs, one taken through a red 
glass, a second through a green glass, a third through a blue glass, the final 
positives being backed respectively by red, green, and blue, an image in natural 
colours could be obtained. In this case also he expresses the view that the several 
colour sensations are most stimulated by the wave-length, which most nearly repre- 
sents the sensations, a statement which is, as we will presently see, erroneous. 
I think, therefore, that Maxwell, while correctly defining the theoretical con- 
ditions governing the choice of light filters for obtaining the negatives, yet 
having himself an erroneous notion as to the rays which cause the maximum 
stimulation of the primary colour sensations, fell into a logical error in practising 
his own invention. 
For many years after Maxweli’s time triple, quadruple, or even more manifold 
projection were suggested by various writers; notably by Collen, Ducos du 
Hauron, and von Ransonnet. Maxwell’s correct theoretical ideas seem to have 
been entirely unknown to these writers, and were not indeed revived till 1886, 
when Mr. Ives of Philadelphia proposed just such a method, and applying 
modern resources showed how fully justified was Maxwell’s prediction that, with 
improved plates, the representation of colours could be greatly improved. To 
Mr. Ives is also due the credit of clearly defining how the ‘ taking” screen 
should be chosen. It must be observed that to Professor H. W. Vogel’s great 
discovery in 1873, of the action of certain analine dyes in remedying the colour 
blindness of the salts of silver, the possibility of advance in this direction is 
entirely due. 
As what follows involves an application of Maxwell’s principles in a new 
* Trans. Roy. Soe. Edinb., vol. xxi., pt. a. 
