375. Stop, in lens, 244; iris, 249; use of, 249. Spectrum, colours of, 230. Speed governors, 67, _68_, _69_; Hartwell, 70; marine, 71. Speed of motor car, 110; railway, 187; vacuum, 189, _190_, _191_; Westinghouse, 194, _195_, _197_; George, 194. Wheatstone needle instrument.] Probably the best-known needle instrument is the verse.