In-line with theory, an empirical measurement, for example, for melamine-formaldehyde spheres in argon gas, gives δ δ = 1.26 ± ± 0.13 as measured by one method, and δ δ = 1.44 ± ± 0.19 by another method, as reported by the same authors in the same paper. According to Epstein himself, Millikan found δ δ = 1.154 for oil drops, whereas Knudsen found δ δ = 1.164 for glass spheres.