|The instrument maker John Dollond announced the design of this micrometer in 1753. It quickly became the standard device for accurately measuring small angles and was included as an accessory in transit expeditions.
In use, it was fitted to the object (rather than the eye) end of a telescope. Its lens is split into two halves which, when moved out of alignment, produce two separate images. By bringing two distinct points into alignment their separation can be measured.