48 
POPULAR SCIENCE REVIEW. 
parent, and the colouring matter readily sets off upon another 
sheet which forms its permanent abiding place. The first 
portion of the picture is laid down upon this final support, and 
by sponging the back of the transparent impressed paper with 
alcohol the image leaves the latter and adheres to the former 
sheet. The second picture is now laid down in a similar 
manner ; and the temporary support being transparent, correct 
registration is secured by moving the second picture until each 
colour is seen to fall into its proper place. Finally, the third 
colour print is treated in the same way, and the completed 
picture contains every portion of the subject, although printed 
by three different operations. The mixture of colours may be 
practised to any desired extent ; thus, if green is to be printed, 
the corresponding portion would be stopped out of the red-print 
negative only, and so with the other secondary colours purple 
and orange. 
The foregoing was, in effect, the description which the writer 
of this article gave of M. Vidal’s process of photo-polychromy 
at the British Association, and the specimens which he exhibited, 
showing the production of a picture in all its stages, were re- 
ceived with a marked degree of interest. The process certainly 
is ingenious, and the prints are very brilliant. They have been 
compared to chromo-lithographs, but although produced in a 
manner somewhat analogous, there is this important difference 
between them, viz., that while in the latter there is no real 
gradation of tone, the chromo-photograph possesses all the 
delicacy of shading and tint peculiar to a photographic print. 
But as the production of each tint is the result of an exposure 
of several minutes to the light of the sun in a printing frame, 
it follows that speed in the production of these prints can never 
be attained. If rapidity be desired, a method of printing 
similar to that of lithography must be employed, and fortunately 
in the now extensively adopted system of “ heliotypy ” or 66 col- 
lotypy” we have that very means at disposal for producing 
photo-heliochromes in a manner far more rapid and equally 
effective than that of M. Vidal. We shall present an outline of 
a modified process of this kind by which we have seen a number 
of coloured prints of great merit produced by an ordinary 
Albion printing-press, and it is very probable that this process 
will soon be extensively adopted for commercial purposes, for 
this reason, that while the proofs possess all the brilliance of 
chromo-lithographs, they further possess the accuracy and 
beautiful gradation of photographs. 
The negatives are prepared in the same way as those of M. 
Vidal, viz., there must be as many negatives as there are colours 
to be produced, and from each negative must be stopped out 
every part that does not represent one special colour. Suppose 
