"...represents two parabolic reflectors, mn and op. It is a peculiarity of such reflectors that rays…
Bilingual Counting Card featuring illustrations of two Sailfishes.
Bilingual Counting Card featuring illustrations of two Sandcastles.
English Counting Card featuring illustrations of two Sandcastles.
Spanish Counting Card featuring illustrations of two Sandcastles.
"Partly fill two bottles with water. Connect them by a bent tube that fits closely into the mouth of…
Bilingual Counting Card featuring illustrations of two Sponge Divers.
English Counting Card featuring illustrations of two Sponge Divers.
Spanish Counting Card featuring illustrations of two Sponge Divers.
Bilingual Counting Card featuring illustrations of two Starfishes.
"A simple telegraph consisting of two telephone receivers, two batteries, and two keys." -Avery 1895