IRIDESCENT COLORS OF BIRDS AND INSECTS—-MALLOCK. 4927 
low intensity is of course due to the small area of each convex surface 
which reflects light in any given direction. 
In attempting to investigate the origin of the colors many methods 
were employed, the first and most obvious being to cut thin sections 
normal to the color-producing surface and then to examine them 
with the highest microscopic power available. If the colors are 
analogous to those of thin plates, it is clear from the high intensity 
of the reflected light that more than one pair of surfaces must cooper- 
ate in the reflection. In general the reflected light is not even 
approximately monochromatic, and this fact limits the number 
of surfaces which can be supposed to act, but if the surfaces are 
supposed to be separated by air and placed at the most favorable 
intervals their number need not exceed three or four to account 
for the observed intensity and tints. 
The most favorable spacing for the successive layers is that their 
thickness and the intervals between them should be a multiple of the 
half wave-length of the mean ray, reckoned in the length of the waves 
within the material of the layer, and it was thought possible that the 
thin sections might show a laminated structure. 
For the material of feathers and insects’ scales, ~ is somewhere 
about 1.5 or 1.6, so that the least thickness for the plates of refractive 
material would be of the order of one one-hundred-and-fifty-thou- 
sandth and the air intervals one one-hundred-thousandth of an inch— 
both beyond the resolving power of the microscope; but from the 
composition of the reflected light it seemed likely that the intervals 
might be two or three half wave-lengths, which would be readily seen 
as far as adequate separation of the images is concerned. In nearly 
all the sections examined bands of this order of thickness appeared 
with some forms of illumination, but it was impossible to be sure that 
they were not due to diffraction effects from parts of the section slightly 
out of focus. 
There are many difficulties in preparing sections thin enough for the 
advantageous use of objectives with large angular aperture. When a 
section is to show a stratified structure its thickness should certainly 
not be greater than the distance between the successive strata, and 
may with advantage be much less. It was not difficult to cut sections 
about one twenty-thousandth of an inch thick, but this is three or 
four times too thick to show with certainty stratifiention whose pitch 
is one sixty-thousandth or less. 
Occasionally, by accident, thinner sections (perhaps one forty- 
thousandth) would be cut, and these showed apparent stratification 
most plainly, but in no case was the image free from the effect caused 
by some part of the thickness of the section being out of focus, and, in 
all probability, what appeared to be stratification was in reality a series 
