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Comment by gameshot911 | Hacker News Reader

Another cool fact is that the human eye can see individual cells! [1] In this example it's a white blood cell, but the eye can also see human ovums as well.

I was convinced this was the case as a kid, but no one ever believed me!

[1] https://www.wikiwand.com/en/Blue_field_entoptic_phenomenon

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One time a fellow intern and I were isolating single colonies of spirulina (many bright green algae* cells in a spiral [1], which is roughly 50 micron wide and maybe 15 micron tall, if memory serves), and I got one colony on it's own in a drop of water under a microscope (using a special pipette & the wick effect for suction).

I took my eyes off of the eyepiece, and looked at the drop of water. I could see a bright green dot in the middle, right where I'd left my spirulina.

I asked the other intern if they saw it too; they did. It was a magical moment.

I had a similar experience when I was doing something with H. pluvalis (an algae that turns bright red and forms a film under stress [2], you've likely seen it in a birdbath or shallow puddle) and I put this film under the microscope, looked at it, and saw it was 1 cell thick. I'm confident if I had gotten a single one of those cells on it's own, I could've seen it. Can't confidently remember how big they were, but large on the scale of cells you find in a drop of water (maybe 25 micron in diameter?), but smaller than most animal cells.

* Technically a cyanobacteria and not an algae, but if a lay person looked at a pond of it they'd say to themselves, "that's a bunch of algae"

[1] https://images.fineartamerica.com/images-medium-large/1-spir...

[2] https://images.squarespace-cdn.com/content/51511761e4b0323b0...

There are single-cell organisms up to an inch or so in diameter. https://en.m.wikipedia.org/wiki/Valonia_ventricosa

There are even visible bacteria! https://www.science.org/content/article/largest-bacterium-ev...

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