Figure 26. The first new design of goggle for the RAF in WWII, the Mk IV. That its ancestry was the earlier folding split lens goggle types is perhaps best shown by the hinged ‘window’ frames of the first Mk IV model. As they were supposed to be worn continually by pilots the concertina fold-ability of the earlier soft mask versions was deliberately missing. The ‘lenses’ were officially called ‘windows’ by the RAF; as the term lens intrinsically implies a distortion of light in some way. (image www.historicflyingclothing.com)