The velocity is not the same for all the corpuscles, nor indeed could we expect that it should be: for as Ladenburg has shown (_Ann. der Phys._, 1903, 12, p. 558) the seat of their emission is not confined to the surface layer of the illuminated metal but extends to a layer of finite, though small, thickness. Thus the particles which start deep down will have to force their way through a layer of metal before they reach the surface, and in doing so will have their velocities retarded by an amount depending on the thickness of this layer. The variation in the velocity of the corpuscles is shown in the following table, due to Lenard (_Ann. der Phys._, 1902, 8, p. 149). +------------------------------------+--------+----------+-----------+ | | Carbon.| Platinum.| Aluminium.| +------------------------------------+--------+----------+-----------+ | Corpuscles emitted with velocities | | | | | between 12 and 8 × 10^7 cm sec. | 0.000 | 0.000 | 0.004 | | between 8 and 4 × 10^7 cm sec. | 0.049 | 0.155 | 0.151 | | between 4 and 0 × 10^7 cm sec. | 0.67 | 0.65 | 0.49 | | | | | | | Corpuscles only emitted with the | | | | | help of an external electric | 0.28 | 0.21 | 0.35 | | field. +--------+----------+-----------| | | 1.00 | 1.00 | 1.00 | +------------------------------------+--------+----------+-----------+ If the illuminated surface is completely surrounded by an envelope of the same metal insulated from and completely shielded from the light, the emission of the negative corpuscles from the illuminated surface would go on until the potential difference V between this surface and the envelope became so great that the corpuscles with the greatest velocity lost their energy before reaching the envelope, i.e. if m is the mass, e the charge on a corpuscle, v the greatest velocity of projection, until Ve = ½mv². The values found for V by different observers are not very consistent. Lenard found that V for aluminium was about 3 volts and for platinum 2. Millikan and Winchester (_Phil. Mag._, July 1907) found for aluminium V = .738. The apparatus used by them was so complex that the interpretation of their results is difficult.