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 suppose 

 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. 



