To observe the position-angle of a double star it is only necessary to turn the position-web so that it shall be parallel to the line joining the centres of the components of the double star.
In addition to Arcturus, the brightest in the group, the most interesting stars of this constellation are: e Bootis, a beautiful double star composed of a yellow star of magnitude 3, and a blue star of magnitude 62; Bootis, a double star composed of a yellow star, magnitude 41, and a purple star, magnitude 61-; and W.
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.
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.
In 1812 Bessel measured with it the angle between the components of the double star 61 Cygni and observed the great comet of 1811.
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.