Optically Worked Double Concave Glass Lens, 75mm Diameter, 150mm Focal Length Category_Science Gifts Square-topped wooden screw simple-machine model

Square-topped wooden screw simple-machine model with a leverage block

two adjustment screws on a spring-loaded aluminum bar

Rotary-knob resistance substitution board offering twelve discrete values from 33 ohms to 10M ohms with 4mm lead connections

9A with oxidized constantan winding on a vitreous-enameled steel tube and a phosphor-bronze sliding contact

Optically Worked Double Concave Glass Lens, 75mm Diameter, 150mm Focal Length Category_Science Gifts Square-topped wooden screw simple-machine modelOptically worked double concave glass lens at 75mm diameter and 150mm focal length, the diverging curvature producing virtual upright images for refraction and beam spreading work.