AFAIK, the advantage of the telescopic sight was, that the crosshairs or ring, or whatever was
shown in the sight, was projected at an infiniet distance, so eliminating the need for the pilots
eye to accommodate. Additionally, the target symbol could be illuminated. Magnification was between
1 and 1.5.
A description of what maybe was the best known type of telescopic gunsight, the "Aldis sight"
can be found on several sites, the original source is Harry Woodman, Aircraft Armament: The
Aeroplane and the Gun up to 1918 (AAP London, 1989) :The Aldis Sight
"In a roughly printed booklet written in 1916 Maj. L.W.B. Rees advocated types of sights that
would be best suited to air fighting. One was for a free gun which involved a rotation arm and
there were comments about telescopic sights. He considered that the magnification had to be
small, as the vibration of the machine would interfere with the sight, and that the field of vision
should be about ten degrees or larger. The eyepiece was to be arranged so that the 'full field of
fire is obtained when the eye is held about a foot away from the telescope; this enables one to
use goggles or a wind screen. So long as the eye is within the angle shown [in a rough sketch]
it need not be in the centre line of the telescope in order to obtain the full field.'
(...) some research involving special optical sights had been undertaken before the war and this
exploratory work continued in the workshops of the Aldis Brothers, of Sparkhill, Birmingham. Their
first effort was a 32in telescopic tube with a graticule engraved on an internal screen; in aerial
fighting magnification was unnecessary and indeed a drawback. After tests at Martlesham in 1915
an improved version of the sight was produced which incorporated all the recommendations (...) it
was approved and the Aldis company was then instructed to commence manufacture of the sight
against an initial order for 200 items as soon as possible.
The Aldis sight consisted of a metal tube 32in long and 2in in diameter. It embodied the priciple of
the ring sight and the sighting system was in the form of two concentric rings engraved on clear glass
screens inside the tube which also contained a number of lenses. When the pilot looked through the
tube the image was neither enlarged nor diminished and was always seen with it's centre directly on
the axis of the sight regardless of the position of the gunner's eye. The rear end was protected by a
rubber sleeve and eyepiece while a problem which had arisen during tests, the fouling of the front lens
of the sight by oil or smoke, was solved by fitting a protective disc which could be raised or lowered by
the pilot. The secret of the Aldis lay in its series of internal lenses and the company always insisted
that sights should not be tampered with and had to be be returned to the factory in the event of
damage or malfunction. According to L.W. Sutherland, writing in Aces and Kings, the reason for this secrecy was that to prevent fogging of the internal lenses certain gases had at the time of
manufacture been introduced between the lenses at varying temperatures and if an Aldis were opened
up these gases would disperse. It was claimed that this was the reason the Germans never copied the
sight dispite the large numbers captured. The German pilots nevertheless liked the Aldis and frequently
fitted it to their machines, the range of optical devices produced by German manufacturers during the
war notwithstanding. The first production Aldis sights were issued to operational units in mid-1916 and
by the end of the year they were being delivered in large numbers for use with the fixed Vickers gun
or the overwing ewis by the RFC and the RNAS. The Aldis remained in service with the RAF until the
late 1930s and the Americans also adopted it, their particular version being known as the "Unit Sight" "
