For the measurement of wider stars he invented his lamp-micrometer, in which the components of a double star observed with the right eye were made to coincide with two lucid points placed io ft.
This angle, therefore, divided by the magnifying power of the telescope gives the real angular distance of the centres of a double star.
I, 2 and 3), used to bolt the head of one of the screws, and the instrument was provided with a slipping piece, giving motion to the micrometer by screws acting on two slides, one in right ascension, the other in declination, so that " either of the, webs can be placed upon either component of a double star with ease and certainty (Mem.
In estimating theoretically the resolving power on a double star we have to consider the illumination of the field due to the superposition of the two independent images.
If the angular interval between the components of a double star were equal to twice that expressed in equation (15) above, the central disks of the diffraction patterns would be just in contact.
