While slides aren't required, they are very, very recommended.
The reason is that high power microscopes need A LOT of light - and the best way to do this is to shine light directly through a thin sample - hence a microscope slide. However, some things lend themselves to being looked at without slides.
Using a phone's main camera works great with the Micro Phone Lens. I will admit it takes a few minutes to become proficient, but it's pretty easy to use one hand to hold your phone and slide with the other free. I'll post a tutorial video later in the campaign.
The reason the videos primarily depict using the front camera is to easier to show the lens and the screen at the same time.
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