length and a rather high voltage.  As a tube of this sort is used, the gas gradually disappears and it must be replenished.  In order to replenish the gas, Moore devised a valve for feeding gas automatically.  An advantage of this mode of light-production is that the color or quality of the light may be varied by varying the gas used.  Nitrogen yields a pinkish light; neon an orange light; and carbon dioxide a white light.  Moore’s carbon-dioxide tube is an excellent substitute for daylight and has been used for the discrimination of colors where this is an important factor.  However, for this purpose devices utilizing color-screens which alter the light from the tungsten lamp to a daylight quality are being used extensively.

The vacuum-tube method of producing light has an advantage in the selection of a gas among a large number of possibilities, and some of the color effects of the future may be obtained by means of it.  Claude has lately worked on light-production by vacuum tubes and has combined the neon tube with the mercury-vapor tube.  The spectrum of neon to a large extent compensates for the absence of red light in the mercury spectrum, with a result that the mixture produces a more satisfactory light than that of either tube.  However, this mode of light-production has not proved practicable in its present state of development.  Fundamentally the limitations are those of the inherent spectral characteristics of gases.  Doubtless the possibilities of the mechanisms of the tubes and of combining various gases have not been exhausted.  Furthermore, if man ever becomes capable of controlling to some extent the structure of elements and of compounds, this method of light-production is perhaps more promising than others of the present day.

There is another attractive method of producing light and it has not escaped the writer of fiction.  H. G. Wells, with his rare skill and with the license so often envied by the writer of facts, has drawn upon the characteristics of fluorescence and phosphorescence.  In his story “The First Men in the Moon,” the inhabitants of the moon illuminate their caverns by utilizing this phenomenon.  A fluorescent liquid was prepared in large quantities.  It emitted a brilliant phosphorescent glow and when it splashed on the feet of the earth-men it felt cold, but it glowed for a long time.  This is a possibility of the future and many have had visions of such lighting.  If the ceiling of a coal-mine was lined with glowing fireflies or with phosphorescent material excited in some manner, it would be possible to see fairly well with the dark-adapted eyes.