Seeing the Unseeable: The Story of the Microscope

Your eye is an extraordinary instrument. But like every instrument, it has limits.

The smallest detail your unaided eye can pick out — viewed from the comfortable reading distance of about 25 cm — is roughly 0.1 mm across. Anything smaller than this gets blurred into a single point.

This is called the limit of resolution of the human eye.

Now consider the typical sizes you'll meet in this chapter:

A typical animal cell is about 0.05 mm across — borderline, even for the very best human eyesight. Most bacteria are far below the limit of vision. To explore the world inside a cell at all, we need help — instruments that can magnify what is too small for the eye, and resolve details that are too close together for the eye to separate.

Such instruments are called microscopes.

In 1665, an English natural philosopher named Robert Hooke built his own simple microscope, capable of magnifying about 200 to 300 times. While experimenting with what he could examine under it, he placed a thin slice of cork — the material from the bark of a particular oak tree, used at the time to seal wine bottles.

What he saw changed biology forever.

The cork was not a smooth solid. Under magnification, it revealed itself to be made of countless tiny, regular, box-like compartments — like rooms in a vast honeycomb. Hooke had no word for what he was seeing. The compartments reminded him of the small bare rooms in which monks lived, called cells in Latin (cella, meaning a small room). So he wrote that name down: cells.

Hooke was looking at the empty walls of dead plant cells — the cellulose chambers that remained after the living matter inside had decayed. He could not have known then that he had given a name to the basic unit of all life. But the word stuck. Today, every biologist on Earth uses Hooke's word.

Hooke's discovery is one of the most powerful examples of what this page's verse describes: when a careful instrument is brought to a careful question, the subtle, hidden, and distant suddenly become visible. The cell had always been there. Hooke is simply the person who first saw it.

A modern light microscope works on the same idea Hooke used: lenses that bend light to enlarge what we see. But it does this with two lenses, not one — and that two-lens design is the secret of its power.

The basic parts of a school light microscope:

• Eyepiece — the lens you look through. Typically magnifies 10×.
• Body tube — the upright cylinder that holds the lenses in alignment.
• Objective lens — a second lens close to the specimen. Microscopes usually have several objective lenses on a rotating turret (e.g. 10×, 40×, 100×).
• Stage — the flat platform where the glass slide carrying the specimen is placed.
• Mirror (or built-in lamp) — directs light up through the specimen so you can see it.
• Coarse adjustment knob — moves the body tube up and down quickly for rough focus.
• Fine adjustment knob — makes tiny precise adjustments for sharp focus.
• Base and handle — give the instrument stability and let you carry it.

The total magnification of a microscope is the magnifying power of the eyepiece multiplied by the magnifying power of the objective lens.

$\text{Total magnification} = \text{Eyepiece} \times \text{Objective}$

With a 10× eyepiece and a 10× objective, the total is 100×. Switching to a 40× objective makes it 400×. The image becomes sharper and more detailed — but also requires more light, finer focus, and a thinner specimen.

A microscope shows you a magnified image of a cell — but how do you work out the cell's actual size? You can't put a ruler under your eyepiece next to the cell.

Here is a clever method, based on careful measurement:

Step 1. Place a transparent ruler (with millimetre markings) on the stage of the microscope. Focus on the markings. Through the eyepiece, you will see a small circular field of view with the ruler's lines crossing it.

Step 2. Measure the diameter of the field of view in millimetres — say, $5$ mm.

Step 3. Convert to micrometres: $5 \text{ mm} = 5{,}000 \text{ μm}$ (since $1 \text{ mm} = 1{,}000 \text{ μm}$).

Step 4. Replace the ruler with your specimen — say, a thin onion peel. Focus carefully.

Step 5. Count how many cells fit along the diameter of the field of view in a single straight line.

Step 6. Apply the formula:

$\text{Estimated size of one cell} = \frac{\text{Diameter of field of view in μm}}{\text{Number of cells along the diameter}}$

Worked example: Suppose you count 25 onion cells across a field of view that is 5,000 μm wide. The size of one onion cell is approximately:

$\frac{5{,}000 \text{ μm}}{25} = 200 \text{ μm}$

This is just an estimate — but a powerful one. With a ruler, a microscope, and simple division, you have measured something far smaller than your eye can see. That is the everyday magic of the scientific method.

• The limit of resolution of the human eye is about 0.1 mm. Most cells are smaller than this — they cannot be seen unaided.

• In 1665, Robert Hooke built his own microscope, examined a slice of cork, and named what he saw 'cells' (after the small rooms of monks). He gave biology its most important word.

• A light microscope uses two lenses (eyepiece and objective). Total magnification = eyepiece × objective.

• An electron microscope uses a beam of electrons (much shorter wavelength than light) and resolves details at the nanometre scale — but cannot work with living cells.

• The size of a cell can be estimated by measuring the diameter of the microscope's field of view and counting how many cells fit across it. The formula stays valid at any magnification.

• Sir Jagadish Chandra Bose invented the crescograph — a magnifying instrument capable of detecting cellular responses in plants — and proved that plant cells respond to stimuli like animal cells do.

• The Yoga Sutras' vision — that focused attention reveals the subtle, the hidden, and the distant — is the same human longing that drives every advance in microscopy.