The ‘Planox Stéréo­scope Mag­né­tique’ was one of the most pop­u­lar French stereo view­ers and is notable for its inno­v­a­tive mag­net­ic mech­a­nism. The basic idea was devel­oped by Mod­este Noel, who reg­is­tered his patents in 1913 and 1914. Alexan­dre Plocq was like­ly the first to use Noel’s inven­tion for pro­duc­tion mod­els. The draw­ings of a third patent in 1920, again by Noel, show the sim­pli­fied mech­a­nism found in the view­ers that were actu­al­ly produced.
The illus­trat­ed view­er is a lat­er ver­sion with bake­lite trays instead of wood­en trays. Due to the design of the bake­lite trays, it was always pos­si­ble to insert both 45x107mm and 6x13cm slides into the same mech­a­nism. How­ev­er, slides in the small­er for­mat need to be stopped at a low­er lev­el than the larg­er ones. There­fore, a thick piece of felt is attached beneath the top lid to stop the move­ment. Behind the opti­cal unit, there’s an addi­tion­al mask­ing-frame for the small­er slides. For pro­duc­tion effi­cien­cy, these two sim­ple mod­i­fi­ca­tions are of course much more attrac­tive than hav­ing to scale the whole mech­a­nism. How­ev­er, this 45x107mm ver­sion also shows a lack of care, because they did­n’t even short­en the slot of the oper­at­ing knob. As a result, it keeps hang­ing in midair.
A much more sat­is­fy­ing solu­tion were the bi-for­mat ver­sions have an addi­tion­al lever to switch between 6x13cm and 45x107mm.