The refractive index is a very important property of the components of any optical instrument. Together with the optical instrument manufacturer Zeiss, he developed the slit ultramicroscope. Sparrow's resolution limit is an estimate of the angular resolution limit of an optical instrument. He was apprenticed to the optical instrument maker James Mann. After middle school, he worked at the Fourth Optical Instrument Factory of Beijing. W. Watson and Son was an optical instrument maker. The sextant, an optical instrument, is used to perform this function. A camera is an optical instrument used to record images. A pseudoscope is a binocular optical instrument that reverses depth perception. An autocollimator is an optical instrument for non-contact measurement of angles. A perspective machine is an optical instrument designed to help artists create perspective drawings. A microdensitometer is an optical instrument used to measure optical densities in the microscopic domain. Phenomenology provides, as Cho often emphasizes, the purest "optical instrument" with built-in possibilities for self-correction.