Ultrasound and railway semaphore: what they have in common

1. Step 1. Ultrasound — is used in — Lens replacement. Phacoemulsification.
2. Step 2. Lens replacement — uses — Lens. An artificial lens is a lens.
3. Step 3. Lens — is used in — Optical telegraph. Signals were read through telescopes.
4. Step 4. Optical telegraph — evolved into — Railway semaphore. The semaphore idea.

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