An exchange of rubber for optical glass was proposed by Britain and Germany during the First World War. Optical glass was vital to the warfare of this era for binoculars and gunsights and rubber was needed for tyres and communications cables. Britain had sourced the majority of its pre-war optical glass from the German company of Carl Zeiss AG and by early 1915 was suffering from a shortage. Germany, with its sea trade blockaded by allied forces, was unable to import natural rubber and found it could not create enough high-quality synthetic rubber to replace it. The British Ministry of Munitions proposed an exchange of British-sourced rubber for German optical instruments through intermediaries in Switzerland. Terms were agreed for tens of thousands of pairs of binoculars to be exchanged b