_Field of Eyepiece._—Observers often require to know the diameter of the fields of their eyepieces. Those engaged in sweeping up comets, nebulæ, or other objects requiring large fields and low powers, find it quite important to have this information. They may acquire it for themselves by simple methods. A planet, or star such as δ Orionis, η or γ Virginis, or η Aquilæ, close to the equator, should be allowed to run exactly through the centre of the field, and the interval occupied in its complete transit from ingress to egress noted several times. The mean result in min. and sec. of time must then be multiplied by 15, and this will represent the diameter required in min. and sec. of arc on the equator. A planet or star near the meridian is the best for the purpose. If the object occupies 1 min. 27 sec. of time in passing from the E. to the W. limit of the field, then 87 sec. × 15 = 1305″, or 21′ 45″. A more accurate method of deriving the angle subtended by the field is to let a star, say Regulus, pass through the centre, and fix the time which lapses in its entire passage by a sidereal clock; then the interval so found × 15 × cosine of the declination of Regulus will indicate the diameter of the field. Suppose for instance, that the star named occupies 2 min. 14 sec. = 134 sec. in its passage right across the whole and central part of the field: then 134 log 2·127105 15 log 1·176091 Dec. of Regulus 12° 30′ log cos 9·989581 ———————— 1962″ log 3·292777 so that the diameter of the field of the eyepiece must be 32′ 42″, nearly corresponding with the diameter of the Moon.