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has observed that for a thin deposit of radioactive matter the distribution of the active discharge about the normal to the surface indicates a uniformity of emission at all angles from any given very small element of volume.Schmidt, 2 on the other hand, has taken into consideration the effect of absorption in the case of a thick film of active substance and finds a resultant effective intensity roughly proportional to the cosine of the angle of emergence of the active particles.In the case of photo-electric emission, it would seem from the work of Stuhlman 3 and Robinson, 4 with thin metallic films, that the activity of the exciting light is not confined to the immediate surface of the metal, but that a finite thickness is effective in the production of photo-electrons.Ladenberg 5 concludes that the effective light penetrates to a depth of eight wave-lengths.In his Physical Optics, Wood has given figures indicating that throughout the ultra-violet region a large percentage of the incident light is absorbed by metals.On account of their high velocities, the (3 and cathode particles are able to penetrate a considerable distance through absorbing media.For photo-electrons, Lenard 6 has recorded velocities ranging from io 7 to io 8 cm. per second from carbon, platinum, and aluminum.The penetrating power is thus much less than that of /3 and cathode particles, but no doubt the photo-electrons emerge from a finite thickness of metal and present a similar case to that of the emission from thick films of radioactive matter.The velocity distribution curves of Richardson and Compton, 7 considered in connection with their theory, would also indicate a falling off in velocity due to a penetration of varying thickness of material.
Willard Gardner (Sat,) studied this question.