34 Mr. A. H. Pfund: A Study 
zine, and iron were made: but upon investigation it was 
found that the maximum had not been shifted. Thinking 
that some impurity, always present, was the cause of this, I 
purified the selenium chemically, added selenides of copper, 
silver, mercury, lead, and used carbon electrodes. However, 
the maximum of the curve was not shifted. In these experi- 
ments the only substance present in every case was selenium, 
and it appears to me that, after all, this is probably the 
determining factor rather than the selenide. In view of 
the fact that the amount of selenide added to the cell 
was approximately that necessary to produce highest 
sensibility, the conditions favourable to a shift in the 
maximum could not have been better. The persistence of 
the maximum in the same position indicates that the nature 
of the metal in the selenide does not control the selective 
sensibility of the cell. 
Any attempt to couple this phenomenon with the optical 
properties of amorphous selenium has thus far been unsuc- 
cessful. As far as the reflecting and transmitting power of 
selenium is concerned, the facts are that a very thin layer 
of amorphous selenium, which is practically opaque to violet 
and ultra-violet rays, transmits red and infra-red rays with 
great readiness, and has no absorption-band in the region of 
maximum sensibility of the selenium cell. The reflecting 
power is at a maximum in the greenish yellow, decreasing 
gradually with light of increasing wave-length. These 
results do not seem to warrant the forming of any definite 
conclusions. However, as we are dealing not with amor- 
phous but with metallic selenium, and as we know but very 
little about the latter, the question must be considered as 
still open. — “3 
While it is difficult to determine the optical constants of 
amorphous selenium, it is doubly so to make similar deter- 
minations for metallic selenium. For example, an attempt 
was made to convert a film of selenium which had been 
deposited upon glass by cathode discharge, into the metallic 
modification. Originally the film had a pale orange colour, 
being perhaps less than the wave-length of violet light in 
thickness, but during conversion it lost its colour, appearing 
finally as a uniform dark grey film, which permitted but 
little light to pass through. A film, thicker than the first 
but still easily transparent to red light, was also converted 
into the metallic modification, with the result that it had 
become absolutely opaque, had received numerous cracks, and 
had drawn itself away from the glass completely. However, 
by pressing out some molten selenium between two glass 
. il 
