Long tubes were certainly employed by Arabian astronomers, and very
probably also by the Greeks and Romans; the exactness of their
observations being in some degree attributable to their causing the
object to be seen through diopters or slits. Abul Hassan speaks very
distinctly of tubes, to the extremities of which ocular and object
diopters were attached; and instruments so constructed were used in
the observatory founded by Hulagu at Meragha. If stars be more easily
discovered during twilight by means of tubes, and if a star be sooner
revealed to the naked eye through a tube than without it, the reason
lies, as Arago has truly observed, in the circumstance that the tube
conceals a great portion of the disturbing light diffused in the
atmospheric strata between the star and the eye applied to the tube.
In like manner, the tube prevents the lateral impression of the faint
light which the particles of air receive at night from all the other
stars in the firmament. The intensity of the image and the size of the
star are apparently augmented.--_Humboldt’s Cosmos_, vol. iii. p. 53.