Tachistoscope "This is a form of short-exposure apparatus. The essential idea is to furnish a field upon which the subject may for a moment fasten his attention, and then to substitute for this field another containing certain prepared test-material. This last field is exposed for but a brief instant and removed, and the subject is then called upon to report all that he has seen during the last exposure. Tests of this kind have demonstrated that the range of visual attention is a comparatively constant quantity with each individual, having but little relation to general ability or intelligence and being but little affected by practice." -DRIVING POWER OF THOUGHT By WARREN HILTON, A.B., L.L.B. FOUNDER OF THE SOCIETY OF APPLIED PSYCHOLOGY(1920),Page 107 source