Viewing slits and tank: what they have in common

1. Step 1. Viewing slits — is part of — Zoetrope. Slits show the frames one at a time.
2. Step 2. Zoetrope — is made of — Paper. The pictures are on a paper strip.
3. Step 3. Paper — is material for — Photometer. A photometer compares the illumination of paper screens.
4. Step 4. Photometer — uses — Reflection of light. Brightness is judged from reflected light.
5. Step 5. Reflection of light — is used in — Periscope. Reflection.
6. Step 6. Periscope — is part of — Tank. Observation devices.

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