The mercury vapor light, developed by American inventors, which in the form of long, glowing tubes, produces more light at less cost than any other practical method of illumination, and would be extensively used if it were not for the color of the light. It is strongest in the violet end of the spectrum, extending far beyond the limits of visibility in that direction and including an abundance of rays that can be photographed but not seen with the naked eye. This mercury glow light has made the "while you wait photography" of the pleasure parks a possibility, but it is too ghastly for common use. But recently German chemists have overcome this difficulty by putting into the electrodes other metals besides mercury, thus changing the character of the light. Zinc with 10 per cent of bismuth and a trace of sodium is used for this purpose.