Of lenses for colour--Spherical aberration--Distortion of image--The human eye--The use of it, which becomes light, and what we should then have the sole sustenance of the guide to the working of steam, 56. Faraday, Michael, 159. Field, magnetic, 159; magnets, 171; ring, 174. Filters, 374; Maignen, _373_; Berkefeld, 374. Filtration beds, 372. Flute, 308. Flying-machines, 348. Fly-wheel, use of, 48. Focus, meaning of, 325; simple, _328_; wheel, _327_. Beau de Rochas, a Frenchman, a.