The visual examination of the ultra-violet end of the
spectrum has been made possible by the ingenious device of
Professor Liveing, who suggested the use of an eye-piece con-
taining a substance which fluoresces visible rays when acted
on by invisible ultra-violet rays. A Liveing eye-piece is,
therefore, essential to the visual study of ultra-violet lines.
When a substance is capable of absorbing light rays of a
certain wave-length, and of emitting them as rays of less



74 THE SPECTROSCOPE

wave-length, it is said to caloresce, but our knowledge of
calorescence at the time is limited.
Phosphorescence may be described as a kind of continuous
fluorescence, for if a phosphorescent substance receives light,
and absorbs it, it emits light of certain wave-lengths, which
emission continues after the illuminating source is removed.
In the case of the phosphorescent bacteria, the emission of light
is due to energy transformed by the organisms themselves.
The phosphorescence seen in the sea is caused by light-
producing infusoria, bacteria, etc., and the latter are chiefly
motile bacilli or vibrios. Access to ox5'gen is absolutely
essential to the bacteria, otherwise they do not phosphoresce.
The light they produce is, therefore, caused by the surplus
energy of respiration. The raj-s given off by Bacillus
phosphor escens, Bacillus luminosus, etc., are chiefly bluish,
but vary between D and G.
A micro-organism test for oxygen has resulted from the
spectroscopic experiments of Engelmann : bacteria essentially
aerobic were placed in water containing green algal filaments,*
and illuminated by the projection of a spectrum caused by
an arrangement of prisms in the sub-stage of the microscope.
