finger 18 is resiliently biased in the direction of the arrow f1 of fig3 and 5 towards the film thus when the film is located in the position shown in fig2 the finger 18 is in engagement with one of the holes 16 when the viewer is then actuated for viewing another pair of frames the finger 18 is displaced in the direction of the arrow f5 of fig2 to disengage the hole and the film driven in the direction of the arrow f3 of fig2 to 5 as the film starts to move the finger 18 is released so that it returns under its bias into engagement with the film however since it is not in alignment with a hole 16 the film moving in the direction of the arrow f3 slides past the finger when the subsequent hole 16 becomes aligned with the finger 18 the finger moves in the direction of the arrow f1 into engagement with the hole whereupon further displacement of the film is halted at this point a fresh pair of frames are now located in the viewing position in the viewer as shown in fig5 the viewer has a casing 20 housing two pairs of mirrors 22 and 24 in operation.