SUMMARY : During normal movement most motile bacteria carry a straight tail, which, when the movement slows, stiffens into helical structures commonly called flagella. The helices of many kinds of bacteria were photographed with a sunlight darkground microscope, and their wavelengths measured. Mean values and standard deviations were calculated for eacli strain and then for the species. 'Biplicity' (two wavelengths per bacterium, one twice the other) was observed frequently. Each strain appears to have its own constant wavelengths. The wavelength differs in different kinds of bacteria from 0.60 to 5058p., the distribution over the various species not revealing a distinct pattern nor any obvious correlation with other characteristics. The wavelength is affected by temperature, pH value, and colloid content of medium. These features, and the effects of drying, make stained preparations useless for measuring.

SUMMARY:In sunlight dark-ground microscopy the flagella of a packet of motile sarcina appear as a smooth tail. The tails often stiffen into helices of two different wavelengths, one being twice as long as the other. The short and the long wavelengths are identical in Sarcina ureae and S. agilis. This phenomenon of two wavelengths, one twice as long as the other, was also found, though less frequently, in certain other bacteria. Otherwise the wavelengths of the flagella of these bacteria appeared specific.

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