Reflection of light and electrical telegraph: what they have in common

1. Step 1. Reflection of light — is used in — Reflecting telescope. A mirror causes no chromatic aberration.
2. Step 2. Reflecting telescope — is a kind of — Telescope. A telescope.
3. Step 3. Telescope — is used in — Optical telegraph. Signals were read through a telescope.
4. Step 4. Optical telegraph — evolved into — Electrical telegraph. The electrical telegraph replaced semaphore lines.

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