190 Orthochromatic Photography. [Sess. 
IX.—ORTHOCHROMATIC PHOTOGRAPHY. 
By Mr T. CUTHBERT DAY. 
(Read March 27, 1901.) 
THE word “orthochromatic,” as applied to photography, is 
liable to some misapprehension. It means, in its strictest 
sense, the reproduction of an object in its true colours by 
means of photography; but it has now come to be under- 
stood as the correct rendering of an object in monochrome 
by photography, so that the luminosity values of the differ- 
ent colours are correctly reproduced in monochromatic tones. 
To produce photographs in natural colours is an extremely 
difficult operation, which is now distinguished by the name ~ 
of Trichromatic photography, and requires a person with 
considerable skill and experience in order to produce any- 
thing like a presentable result. Orthochromatic photography, 
on the other hand, as dealt with in this paper, is a compara-— 
tively easy process, full of interest, and when taken up 
intelligently by any one accustomed to the use of dry — 
plates, it is capable of affording very pleasing results with — 
little extra trouble, and will be found by all who once 
study it to possess an attraction quite its own. It is my 
intention, to-night, to show what can be done in this way, — 
—first giving, as briefly as may be, the why and wherefore 
of the process. ; 
In order to clearly understand our subject, it will be- 
necessary to refer shortly to a few facts with regard to that 
mysterious agent, Light. The observed phenomena respect-— 
ing light can be fairly well explained by assuming, in the 
first place, that all space, even that occupied by solid and — 
liquid bodies, is permeated by a subtle something to which | 
we give the name of Ether. Certain manifestations of force, 
such as intense chemical action, as we find it in the sun, 
are capable of setting up vibrations in the ether which are 
propagated in all directions. The vibrations, when inter- 
cepted by appropriate substances, can be made to yield | 
energy in the form of heat, light, electricity, and chemical 
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