Another complication arises in the variation in visibility or luminosity of energy of wave-lengths within the range of the visible spectrum.  Obviously, no amount of energy incapable of exciting the sensation of light will be visible.  The energy of those wave-lengths near the ends of the visible spectrum will be inefficient in producing light.  That energy which excites the sensation of yellow-green produces the greatest luminosity per unit of energy and is the most efficient light.  The visibility or luminous efficiency of radiant energy may be ranged approximately in this manner according to the colors aroused:  yellow-green, yellow, green, orange, blue-green, red, blue, deep red, violet.

Newton, an English scientist, first described the discovery of the visible spectrum and this is of such fundamental importance in the science of light that the first paragraph of his original paper in the “Transactions of the Royal Society of London” is quoted as follows: 

In the Year 1666 (at which time I applied my self to the Grinding of Optick Glasses of other Figures than Spherical) I procured me a Triangular Glass-Prism, to try therewith the celebrated Phaenomena of Colours.  And in order thereto, having darkened my Chamber, and made a small Hole in my Window-Shuts, to let in a convenient Quantity of the Sun’s Light, I placed my Prism at its Entrance, that it might be thereby refracted to the opposite Wall.  It was at first a very pleasing Divertisement, to view the vivid and intense Colours produced thereby; but after a while applying my self to consider them more circumspectly, I became surprised to see them in an oblong Form; which, according to the receiv’d Law of Refractions, I expected should have been circular.  They were terminated at the Sides with streight Lines, but at the Ends the Decay of Light was so gradual, that it was difficult to determine justly what was the Figure, yet they seemed Semicircular.

Even Newton could not have had the faintest idea of the great developments which were to be based upon the spectrum.

Now to return to the peculiar property of rare-earth oxides—­namely, their unusual brilliance when heated in a flame—­it is easy to understand the reason for this.  For example, when a number of substances are heated to the same temperature they may radiate the same amount of energy and still differ considerably in brightness.  Many substances are “selective” in their absorbing and radiating properties.  One may radiate more luminous energy and less infra-red energy, and for another the reverse may be true.  The former would appear brighter than the latter.  The scientific worker in light-production has been searching for such “selective” radiators whose other properties are satisfactory.  The rare-earths possess the property of selectivity and are fortunately highly refractory.  Welsbach used these in his mantle, whose efficiency is due partly to this selective property.  Recent work indicates that much higher efficiencies of light-production are still attainable by the principles involved in the gas-mantle.