British Military binocular X5 Bino.Prism. Mk IV Ross 1941
The Manufacturer is Ross. The Model is " X5 Bino.Prism Mk IV 5 X 40"
The "B" marking on right prism cover indicates the optics are "bloomed", i.e. coated, and the marking "6E/383" is a Royal Air Force stores code for this type of instrument. This is a fixed focus aircraft spotting and maritime reconnaissance including U-boat spotting binocular. It is reported but not confirmed that it was used by observers in Beaufighters and other nightfighters during WW II (Seeger, page 214). If so, this explains the purpose of the large rubber eyeshields which would be helpful in maintaining night vision but which are, otherwise, awkward to use. The Mk. IV was built by Ross under Royal Air Force contract, introduced in 1941, and continued to be used at least into the 1960's. It was succeeded by the 5X40 6E/392. It is the first British war-time binocular to employ coated lenses. According to a 1943 report by the Admiralty Research Laboratory, the X5 Mk. IV's 8 mm exit pupil gave10%, 15% and 25% "improvements" (in light transmission?) under respectively half-moonlight, starlight and dark conditions compared to a 7X50 Admiralty Pattern 1900 (probably a Barr & Stroud CF41). Mention is not made if the X5 had coated optics which it probably did and which would have improved light transmission.
It could be filled with dry air to prevent condensation and fungus. Porro II design with prisms cemented to increase light transmission by reducing number of air/glass surfaces. Unlike the Ross Bino.Prism No.'s 5 and 6 and the Barr & Stroud CF 41, the ocular field lens is not cemented to prism group. The performance of this binocular is enjoyable. Although it is a fixed focus design, it is possible with effort to adjust the focus to suit one's eyesight (this must be done to fully realize the binocular's optical potential). Once correctly focused, the view is sharp to about 65%-70% with good brightness, color and contrast. Also, the 8 mm exit pupil is comfortable to look through.