1.The shutter eyepiece has been evolved to enable accurate observations to be made for longitude with a field instrument. A main source of inaccuracy in time observations, on which longitude determination depends, has always been the estimation of the time at which a star crosses a line in the eyepiece of the instrument. There are considerable personal differences, as between various observers, in this estimate, in addition to variations of an individual observer; and, further, star motion is somewhat irregular owing to erratic refraction. These difficulties have been overcome to a large extent by the use of the moving-wire micrometer, with which the observer endeavours to keep the star intersected for a considerable time, during which the instants at which the wire crosses certain points is recorded on a chronograph by electrical means. The moving-wire micrometer has been fitted to transit instruments and employed only for meridian transit observations; and it requires a good chronograph. It has not been available for use with a theodolite and is altogether too bulky in its ordinary form for such a purpose. 2. The shutter principle . The principle of the shutter eyepiece is quite different. In this a small central portion of the telescope field is obscured by an opaque body or shutter ( vide fig. 1), located between the object glass and its focal plane, and as close as practicable to the latter. Its object is to expose a star for observation for an instant at a time controlled by clock or chronometer. The shutter is carried on a rocking arm which allows the shutter a lateral movement of amount slightly greater than its width. Movement of the rocking arm is caused by an electromagnet, controlled by a break-circuit chronometer or clock. Thus the exposures are in constant relationship with the time-keeper. For this purpose a system of relays has been contrived, whose functions are: