W. LeConte Stevens—Recent Progress in Optics. 379 
blue, can be made sensitive to waves of much longer period 
than those hitherto effective in photography. In 1885 he 
proposed to sensitize plates for each cf a number of successive 
regions in the spectrum, and to make as many complementary 
pigment prints as negatives, which should then be superim- 
posed. This somewhat complicated plan proved difficult in 
practice. In 1888 F. E. Ives* of Philadelphia, adopting the 
more simple Helmholtz-Maxwell modification of Young’s theory 
of color, applied it to the preparation of suitable compound 
color screens which were carefully adjusted to secure corre- 
spondence with Maxwell’s intensity curves for the primary 
colors. The result was a good reproduction of the solar spec- 
trum. But to reproduce the compound hues of nature it is 
necessary specially to recognize the fact that although the spec- 
trum is made up of an infinite number of successive hues, the 
three color sensations in the eye are most powerfully excited 
by combinations rather than simple spectral hues. Thus, 
according to Maxwell’s curves the sensation of red is excited 
more strongly by the orange rays than by the brightest red 
rays, but the green sensation is excited at thesame time. This 
fact has to be applied in the preparation of the negatives, 
while images or prints from these must be made with colors 
that represent only the primary color sensations. Properly 
selected color screens must therefore be used for transmission 
of light to plates sensitized with suitable aniline dyes; and the 
adjustment of ratios with this end in view is not easy. But it 
has been successfully accomplished. From three negatives 
thus made, each in its proper tint, positives are secured; and 
these are projected, each through its appropriate color screen, 
to the same area upon a white screen. The addition of lights 
thus sent from the triple lantern gives the original tints with 
great fidelity. 
Mr. Ives has devised a special form of camera by which the 
three elementary negatives are taken simultaneously, and also 
an instrument, the photochromoscope, in which a system of 
mirrors and lenses brings to the eye a combination similar to 
that projected with the triple lantern. A double instrument 
of this kind forms the most perfect type of stereoscope, bring- 
ing out with great vividness from the prepared stereographs 
the combined effect of color, form, and binocular perspective. 
It is only within the past year that these improvements have 
been perfected. By further application of the same principles, 
Mr. Ives has produced permanent colored prints on glass, 
which do not require to be examined with the aid of any 
instrument. Each of three negatives is made with a colored 
screen which transmits tints complementary to those which it 
* Journal of the Franklin Institute, January, 1889 
