The luminous efficiencies of arc-lamps depend upon so many conditions that it is difficult to present a concise comparison; however, the following may suffice to show the ranges of luminous output per watt under actual conditions of usage.  These efficiencies, of course, are less than the efficiencies of the arc alone, because the losses in the mechanism, globes, etc., are included.

Lumens per watt
Open carbon arc                             4 to  8
Enclosed carbon arc                         3 to  7
Enclosed flame-arc (yellow or white)       15 to 25
Luminous arc                               10 to 25

Another lamp differing widely in appearance from the preceding arcs may be described here because it is known as the mercury-arc.  In this lamp mercury is confined in a transparent tube and an arc is started by making and breaking a mercury connection between the two electrodes.  The arc may be maintained of a length of several feet.  Perhaps the first mercury-arc was produced in 1860 by Way, who permitted a fine jet of mercury to fall from a reservoir into a vessel, the reservoir and receiver being connected to the poles of a battery.  The electric current scattered the jet and between the drops arcs were formed.  He exhibited this novel light-source on the mast of a yacht and it received great attention.  Later, various investigators experimented on the production of a mercury-arc and the first successful ones were made in the form of an inverted U-tube with the ends filled with mercury and the remainder of the tube exhausted.

Cooper Hewitt was a successful pioneer in the production of practicable mercury-arcs.  He made them chiefly in the form of straight tubes of glass up to several feet in length, with enlarged ends to facilitate cooling.  The tubes are inclined so that the mercury vapor which condenses will run back into the enlarged end, where a pool of mercury forms the negative electrode.  The arc may be started by tilting the tube so that a mercury thread runs down the side and connects with the positive electrode of iron.  The heat of the arc volatilizes the mercury so that an arc of considerable length is maintained.  The tilting is done by electromagnets.  Starting has also been accomplished by means of a heating coil and also by an electric spark.  The lamps are stabilized by resistance and inductance coils.

One of the defects of the light emitted by the incandescent vapor of mercury is its paucity of spectral colors.  Its visible spectrum consists chiefly of violet, blue, green, and yellow rays.  It emits virtually no red rays, and, therefore, red objects appear devoid of red.  The human face appears ghastly under this light and it distorts colors in general.  However, it possesses the advantages of high efficiency, of reasonably low brightness, of high actinic value, and of revealing detail clearly.  Various attempts have been made to improve the color of the light by adding red rays.  Reflectors of a fluorescent red