IRIDESCENT COLORS OF BIRDS AND INSECTS—-MALLOCK. 429 
which interference takes place, the interval between the layers, now 
reckoned in wave lengths in the refractive fluid, is increased, and 
therefore also the wave length which is reflected for a given angle of 
incidence. At the same time the intensity of the reflected light is 
greatly reduced, and if the fluid has the same refractive index as 
the structure itself, reflection ceases and nearly white light is trans- 
mitted.1 
Observation of reflection from films of aniline dyes, etc., shows 
that the color changes in the same direction—that is, toward the 
blue—as the angle of incidence increases, but as regards the character 
of the change when the film is covered by a refractive fluid there is a 
great difference. 
In some cases (methylene green, for instance) for one particular 
angle of incidence the color reflected in air is unchanged when the 
film is covered with cedar oil, for smaller angles of incidence the 
reflected light is of shorter average wave length, and for greater 
angles longer than that of the color in air. 
For this particular dye the color reflected in air is a very red-purple 
at small angles of incidence, changing to bluish-green when the 
angle is large. 
Under cedar oil the colors are respectively greenish-yellow and an 
orange-yellow. The transmitted color, however, does not change 
perceptibly either with the angle of incidence or with the medium 
in which reflection takes place, and this applies, as far as my obser- 
vation goes, to all substances which give selective metallic reflection. 
The transparency, or at any rate the vanishing of the character- 
istic transmitted color in the case of all animal tissues when immersed 
and permeated by a fluid of the same refractive index, is strongly in 
favor of interference being the source of the color, but even stronger 
evidence is given by the behavior of the structure under mechanical 
pressure. 
If the grain or peculiarities which favor the reflection or transmission 
of particular colors is of molecular size, there is no reason to suppese 
that pressure insufficient to cause molecular disruption would alter 
the action of the material on light. On the other hand, if the colors 
are due to interference—that is, to cavities or strata of different 
optical properties—compression would alter the spacing of these, and 
thus give rise either to different colors or, with more than a very 
slight compression, to the transmission and reflection of white light. 
In every experiment of this kind which I have made either on 
feathers or insect scales the effect of pressure has been to destroy 
the color altogether: 
1 The dispersion of the fluid, as well as the refractive index, must be the same as that of the structure 
if the transmitted light is white, 
