Here's how the Aldis sight looks on the FS-WWI SE5a (as a modeled object with a viewable lens):



Here's how the Aldis sight looks in OFF (also as a modeled object with a viewable lens):



Both sim platforms for these WWI mods (SDOE & CFS3) allow cockpit zooming, so that the
pilot's viewpoint can move closer to the sight. Here's a view showing a moderate zoom:



Finally, here's a sight look sitting in a real Sopwith Camel (w/rubber eyepiece attached):



Without magnification, the Le Chretien and Aldis collimate the viewpoint and superimpose a
ring scale and dot onto the target, without causing the pilot to loose his spatial awareness.
(almost like having a "bore-sight" to use in one's frontal view)

Freycinet, the best two books I know of on WWI armament are:

Early Aircraft Armament by Harry Woodman
Flying Guns World War I by Anthony G. Williams & Dr. Emmanuel Gustin

Both books have information on tubular sights, with Woodman having the most. \:\)