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Arthur Hill Hassall (1817– 1894) was a British physician, and a pioneer in the use of the microscope as a tool in medicine and public health. In 1846 he published a two-volume study, ‘The Microscopic Anatomy of the Human Body in Health and Disease’, the first English textbook on the subject.

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The microscope was at the centre of Virchow’s work on disease processes, he would urge his students to 'learn to see microscopically'. Much of Virchow’s work involved investigating tissue and cells in the laboratory and then relating his findings back to clinical changes in his patients.

Not much changed in basic microscope design over the next 200 years, but improvements in lens manufacture (such as the use of purer glass) helped to solve problems like colour distortion and poor image resolution. Mirrors were added to compound microscopes to add more light and improve the image.

Leeuwenhoek observed animal and plant tissue, human sperm and blood cells, minerals, fossils, and many other things that had never been seen before on a microscopic scale.

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Unlike stereo and compound microscopes, which use regular light for image formation, the confocal microscope uses a laser light to scan samples that have been dyed. These samples are prepared on slides, which the device then converts into a magnified digital image. Operators can also create 3D images by assembling multiple scans. They are commonly used in cell biology and medical applications.

Between 1838 and 1839 two German scientists, Mathias Schleiden (1804–81) and Theodor Schwann (1810–82) proposed that cells were the building blocks for plant and animal life.

Compound microscopes have two lenses: the second lens magnifies the image enlarged by the first lens. Modern compound microscopes can provide a magnification of 1,000 times. They are still the most commonly used general purpose microscopes, found everywhere from research labs to school biology laboratories.

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Two main problems hindered lens manufacture: image blurring (spherical aberration) and colour separation (chromatic aberration). Around 1830, Joseph Jackson Lister, in collaboration with instrument maker William Tulley, made one of the first microscopes that corrected for both these faults.

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His second book, ‘A microscopical examination of the water supplied to the inhabitants of London and the suburban districts’ (1850), became an influential work in promoting the cause of water reform.

In the 1660s, another Dutchman, Antonie van Leeuwenhoek (1632-1723) made microscopes by grinding his own lenses. His simple microscopes were more like magnifying glasses, with only one lens.

Many researchers refused to use the early microscopes because they could not trust what they were seeing. Aberrations and impurities in the lenses caused distortions, which led to errors in observations.

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The resolution of a light microscope is typically considered to be the wavelength divided by twice the numerical aperture of the microscope.

It’s not clear who invented the first microscope, but the Dutch spectacle maker Zacharias Janssen (b.1585) is credited with making one of the earliest compound microscopes (ones that used two lenses) around 1600. The earliest microscopes could magnify an object up to 20 or 30 times its normal size.

He was the first person to use the term ‘cell’ to describe what would later be recognised as the building blocks of all living organisms, plant and animal.

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His theory was taken up by another German researcher, Rudolf Virchow (1821–1902), possibly the most influential teacher of pathology in the 1800s.

all perforated and porous, much like a Honey-comb, ... these pores, or cells, ... were indeed the first microscopical pores I ever saw.

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And in the early 1850s he also studied food adulteration, publishing his findings in medical journal The Lancet. The journal's campaign on food adulteration led directly to the 1860 Food Adulteration Act.

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Like the scanning electron microscope, the transmission electron microscope (TEM) uses electrons to create a magnified image, and samples are scanned in a vacuum so they must be specially prepared.

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He made use of the latest developments in microscopy such as the use of microtomes to cut very thin slices of tissue and the development of stains to highlight the parts of a cell.

From the 1830s, cells and cell theory became the focus of medical and biological research, thanks to the central role of the microscope in laboratory science. Researchers were able to describe the body at the microscopic level more consistently and with greater confidence in what they saw.

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Schwann had a medical training and proposed that understanding cellular behaviour was the key to understanding the body in health and illness.

But at the start of the 1800s century, the pioneering French pathologist Xavier Bichat, who carried out many investigations into tissue samples and organs, still refused to use a microscope.

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Hassall’s investigative work with the microscope showed how laboratory science could be used to gather evidence from the field about health and disease.

This image is a transmission electron micrograph showing four views of the Herpes simplex virus, computer-coloured mauve.

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Compound microscope designed by Robert Hooke, 1671–1700, and thought to have been made by Christopher Cock of Covent Garden, London.

The scanning electron microscope, or SEM, uses electrons rather than light for image formation. The sample is dehydrated and coated with a thin layer of a conductive material, such as gold. Samples are placed in an evacuated chamber to be scanned. The SEM produces a black-and-white digital image. SEMs are used by researchers to examine a range of specimens from insects to bones.

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He presented his findings to the Royal Society in London, where Robert Hooke was also making remarkable discoveries with a microscope.

And technological innovations in digital technology improved techniques such as microsurgery, which combines surgery and microscopy to allow detailed and precise manipulations inside the body.

The simple microscope combines a convex lens with a holder for specimens. Magnifying between 200 and 300 times, it is essentially a magnifying glass.

In the 20th century, new instruments such as the electron microscope increased magnification and offered new insights into the body and disease, allowing scientists to see organisms such as viruses for the first time.

Hooke published the ‘Micrographia’ (1665), an astonishing collection of copper-plate illustrations of objects he had observed with his own compound microscope.