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 Telescopes seem to be a subject in which O'Brian was fairly knowledgeable.  Aubrey discusses the virtues of his various telescopes, mentioning the achromatic lenses of his Dollond's, and the relationship between the size of the objective lens and its ability to resolve double stars.  There are frequent references to a night glass, a telescope with fewer optical elements to increase its light gathering capability.  There is also the use of an unusual instrument called a 'Coming Up Glass', for determining whether they are gaining upon or losing ground to a chase vessel.

This was a version of a double image telescope invented by Ramsden. The original purpose was for astronomers to measure small angles, particularly the semi-diameter of the sun and moon. In an otherwise ordinary telescope, one of the lenses was split in half and separated laterally by a micrometer adjustment. With the two halves screwed together a single image was formed. By turning the micrometer screw the two lens halves could be separated, creating an overlapping double image in the eyepiece. The number of turns required to fully separate the two images was a measure of the angle subtended by the object being viewed. Some clever naval officer figured out that this astronomer's tool could be conveniently used for determining whether a distant ship was getting closer or more distant. By first focusing and adjusting the micrometer to get two images just touching, a change in distance would easily be detected by the images either separating or overlapping with time. Really clever!

Don Seltzer