436 JOURNAL, R.A.s. (cbylon). [Vol. XYIIL 
is known as the three-colour or trichromatic principle, and 
which, as its name implies, involves the essentiality of 
three distinct or specially selected colours being compre- 
hended in the composition of the natural colour pictures 
obtained. This was in fact the system originally discovered 
by Prof. Clerk Maxwell of Cambridge University so long 
since as the year 1857, and who, by throwing in correctly 
adjusted combination three distinctly coloured photo- 
graphic images on a lantern screen, produced more or less 
perfect replicas of brilliantly coloured ribbons and other 
objects. 
In its most recent, though probably far from fully per- 
fected, stage of de,velopment it has been found and is be- 
coming generally recognized that there are no colours in 
either Nature or Art that cannot be faithfully reproduced 
in combination with the three-colour or trichromatic photo- 
graphic system. It has been maintained as an explanation 
of this very interesting and well-established fact that all 
colour images are formed on the human retina through the 
medium of three distinct colour-recording nerves, so that 
in the building up of the natural colour photographic 
image we are simply reproducing the conditions that 
obtain in association with normal human colour vision. 
In connection with the development and perfecting of 
this three-colour photographic system several notably in- 
teresting and more or less important modifications of its 
adaptation and application have been devised. In one of 
the earlier of these, with which the name of Mr. Frederick 
Ives of Philadelphia is more particularly associated, the 
images of the three positives obtainei are in their mono- 
chrome form concentrated by means of three distinctly 
coloured mirrors into a single focus within a portable instru- 
ment upon which its inventor conferred the title of the 
" Kromskop." Subjects photographed and viewed through 
this instrument, more particularly in its stereoscopic form, 
present a realistic facsimile of the subject photographed 
that is with difficulty obtained by any other known method. 
