Ophthalmoscope and steam locomotive: what they have in common

1. Step 1. Ophthalmoscope — has part — Lens. Lenses.
2. Step 2. Lens — is part of — Cinématographe. A lens.
3. Step 3. Cinématographe — has part — Crank. The film was advanced by turning a handle.
4. Step 4. Crank — is part of — Steam locomotive. Connecting rods and cranks carry the piston stroke to the wheels.

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