DIFFUSE REFLECTION. 37 



the bolometer, is especially large (perhaps two or three times the usual 

 proportion) in the narrow crescent moon where the rays suffer a grazing 

 reflection at a large angle of incidence, the emission under the correspond- 

 ing angle of emission being small." This is particularly noticeable in the 

 early forenoon. Very's 1 recent discussion gives the impression that, in the 

 moon, "specular reflection" is something to be sought for at the angle of 

 reflection with the sun, at which the image of the sun in rays of 8.5 to 10 /< 

 may be isolated by using a screen with a pinhole aperture. Here "spec- 

 ular reflection" appears to be used in the sense of "regular reflection." 

 The writer is not discussing this limiting case, but it appears self-evident 

 that a matte surface like a plane surface can be selectively reflecting, and 

 hence that a surface of quartz, for example, which, if smooth, reflects like 

 a metal for wave-lengths 8.5 and 9.03 p. and like a transparent medium 

 for all other wave-lengths, must still reflect selectively when it is rough. 2 

 Hence, in the stream of energy reflected in any direction the density will 

 be greatest for wave-lengths 8.5 to 9.03 p.. Of course these bands of 

 "metallic reflection" will now be less intense. Since the energy density 

 in every direction must be greatest for wave-lengths 8.5 to 10 p, one would 

 expect to detect this difference in any direction, and not simply at the 

 angle of regular reflection. 



If, then, the eye were sensitive to the infra-red quartz would have a 

 "surface color" corresponding to wave-lengths 8.5 and 9.03 p. In speak- 

 ing of surface color, however, a sharp distinction must be made, for the 

 reflecting power of these bands is as great as that of metals, although the 

 substance is a non-conductor; whence one would expect the reflecting 

 power to be low. In the case of transparent non-conductors the surface 

 color is due less to reflection than to absorption, for it is due to absorption 

 that the reflected light is deprived of some of its constituents and becomes 

 colored. However, on the long wave-length side of the absorption band 

 the reflecting power is high, which contributes to the color. For example, 

 the aniline dyes, such as fuchsine, have a low reflecting power (as compared 

 with metals) and yet they possess surface color. Pigments belong to the 

 class of substances having "body color." On the other hand, metals, 

 such as gold and copper, also have a surface color, due to selective absorp- 

 tion. They are electrical conductors, however, and theoretically would 

 totally reflect all radiations for all wave-lengths. This has been established 



1 Very: Astrophys. Jour., 24, p. 351, 1906. Here the writer is quoted as having found 

 " that common minerals reflecting diffusively" from 4 to 8 /j. have bands of metallic reflection 

 from 8 to 10 M- 



This quotation is erroneous. The writer found that the reflection was "regular" for all 

 wave-lengths, but that from 4 to 8 /". the reflecting power is low, as in cases of transparent 

 media having low refractive indices, while from 8 to 10 /x the reflecting power is high, like 

 metals, hence called "metallic reflection." 



2 See footnote, p. 146, for experimental evidence supporting these statements. 



