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Oil immersion is an interesting and complex microscopy tool.

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Go to to start streaming thousands of documentaries and for a special offer that helps support our show. If you’ve been following along with us lately, you probably know that we’re pretty excited about our latest microscope upgrades.

And can you blame us? It's like having a new toy that’s actually a spaceship. It’s pretty hard to shut up about something that is shiny and new, and also takes you into a whole new world.

And that’s kind of what it feels like to see our familiar friends at this magnification. Even though we’ve seen tardigrades many times before, in so many different ways, we have not seen them like this. And bringing this level of magnification gives us a new layer of the microcosmos to explore.

It’s like we’ve descended into a new level of a cave, getting further from our world—though fortunately for the easily scared among us, the cave is metaphorical and the risks are negligible. All we need for the journey are some objective lenses and a tiny drop of... oil? Yeah, oil.

But to explain what that oil does, we’re going to have to get a little technical first. Because yes, we love our microbes, but we’re only able to see them thanks to light, and light is complicated, but we need to give it its due. So let’s start with the objective lens.

The main job of the objective lens is to take light leaving the sample and focus it into an image for the us to see. And there are two main numbers that describe the objective lens. The first one’s pretty obvious: it’s the magnification.

But the magnification you see written on the objective lens is not the same as the magnification we write up on the corner of the videos. That’s because the objective lens isn’t the only source of magnification in our microscope. We’re also looking through an eyepiece that has its own magnification.

So the final magnification that we see is the product of the objective lens’ and eyepiece’s magnifications. Now we don’t usually do math on this channel, but this is one of the rare cases where the math and optics is straightforward. In our microscope, the eyepiece has a magnification of 10x.

So if we’re looking through a 63x objective, the final magnification is 630x. And that is the number you’ll see in the corner. But you should never trust an optics lesson that seems simple.

How could it be when the image we’re seeing is the product of so many things at once, some tangible like a lens, others much less so. And what is less tangible than light, which is, as we’ve seen before on this channel--a very strange thing. We cannot touch it, but we can manipulate it, using what we know of the way it travels and bends and reflects to see our world differently.

But light… has its limits. The thing about making things larger is that at some point, it’s not enough to just zoom in. You need to be able to capture the detail that’s there.

And this leads to the second important number you’ll find on an objective lens: the numerical aperture. Unlike magnification, “numerical aperture” is probably not super self-explanatory. What the number describes is the ability of the objective to take in light.

As light travels from the sample and to the objective, it radiates outwards like a cone. The higher your numerical aperture, the wider the cone of light that's going into your objective. And that allows for more rays to enter the objective from all sorts of different angles, helping to illuminate more details and give greater resolution to your final image.

But of course, that’s still not all that goes into capturing the perfect microscopic image. Let’s take a look at this glaucoma spinning around at 1000x magnification. It’s visible, and you can see some of the details.

It looks…fine. But let’s take a look at it again here. Same glaucoma.

Same objective. Same level of magnification. What was “fine” before now just looks dull in comparison to the image we were seeing before with the detailed striations and vivid pockets of green on the glaucoma’s body.

To get from one of these images to the other, James—our master of microscopes—didn’t change any of the technical settings on his microscope or pull some kind of video editing wizardry. He just simply added a drop of oil to the coverslip encasing the glaucoma, and dipped the objective into the oil. Now this is because the cone of light going from the specimen into the objective isn’t just going straight from the sample to the lens.

It’s passing through something—it’s passing through air. Light travels at different speeds through different materials and when it changes from material to material, it will bend. This is called refraction and materials like oil slow light down more.

And that matters to our microscope because it gives us the wider cones of light we need to get a higher numerical aperture. Whether we’ve got air between the sample and the lens or oil when we’re viewing our samples depends on the objective we’re looking through. Our lower magnification objectives have lower numerical apertures, and they’re meant to be used with air.

Our higher magnification objectives have higher numerical apertures, and they’re meant to be used with oil. Using air with oil objectives gives us poor images, and using oil with our dry objective would damage the objective. So…don’t do that.

The numerical aperture and its relationship to resolution is important because it puts a limit on just how much magnification we can actually do with a given objective. There’s a general estimate if you are looking into microscopy yourself, that you’ll get a good magnification when you’re working in the range that’s 500 to 1000 times the numerical aperture of the objective. So our 100x objective and 1.3 numerical aperture can reach a maximum of 1300x magnification depending on your eyepiece.

With our 10x eyepiece, we get 1000x magnification, and it works great. But if we tried to get more from this set-up by increasing the magnification of our eyepiece to 20x, we could not actually get 2000x magnification. Because of the constraints of our numerical aperture, we would just be losing half of our image without actually gaining more detail in what’s left to see.

But that’s what happens any time we want to take an image: we have to make a decision between what we can and cannot see. There are always choices that have to be made, and details that have to be lost. We simply cannot see all of the world, in all its entirety, all at once.

But we can see more of it by tracking the choices we make as we dive deeper, choices in techniques and materials that affect what we can see and how we see it, and enrich the story further, even when those choices impose constraints. At some point in history, we wanted to see more, and lenses with their magnificent manipulation of light have helped us do that. We’ve used them to see light from distant stars, and to peer into the most mundane surroundings on earth.

And whatever image has come back to us has brought the universe closer to view, even if it’s just in fractions. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. This episode was brought to you by CuriosityStream, a subscription streaming service that offers thousands of documentaries and non­fiction TV shows from some of the world's best filmmakers, including award winning exclusives & originals.

They cover topics like history, nature, science, food, technology, travel, and more! There, you can watch the documentary series “Ancient Yellowstone” where scientists discuss how the distribution of microbial life in extreme environments here on our planet can give us a better idea of where in the universe life might be able to exist. You can stream CuriosityStream’s library, including their collections of curated programs handpicked by their experts, to any device for viewing anytime, anywhere, and if you go to and use the word “Microcosmos” to sign up, it will only cost you $14.99 for an entire year!

The people you’re seeing on the screen right now, those are our Patreon patrons. They are the people who help make this show possible. If you like it, they are the ones to thank.

And I am feeling very grateful for them today as they allow us to look deeper, not just in magnification, but also in resolution. If you want to see more from our Master of Microscopes, James Weiss, you can check out Jam & Germs on Instagram and also I think TikTok now. And if you want to see more from us, there’s always a subscribe button somewhere nearby.