The father and son eyeglass makers Hans and Zaccharias Janssen are generally credited with the discovery that, if two lenses are placed in a line with one another, then the light passing through them will be bent and the image of an object will be magnified. This discovery, made at the end of the sixteenth century, soon led to the production of the first simple microscopes.

These instruments consisted of little more than two lenses fitted inside a tube and, at that time, were regarded as toys for the entertainment of the rich. Fortunately, however, one of these toys came into the hands of a very special person: Robert Hooke.

In many ways Hooke was the spiritual successor of Galileo, and he championed the idea of advancing human knowledge by doing experiments in physics and astronomy. But for biologists Hooke's most important discoveries were made looking down a microscope. There, to his surprise and delight, he found a whole new living world, a world no one knew even existed, a world no human eye had ever seen before: the microscopic world of cells.

We know what Robert Hooke saw down his microscope because he published many of his findings in a book called Micrographia: or Some Physiological Descriptions of Minute Bodies made by Magnifying Glasses, with Observations and Inquiries thereupon. Most people refer to this book simply as the "Micrographia."

There are sixty chapters in this book with titles such as "Of an Ant or Pismire," "Of the curious texture of Sea-weeks," and "Of a Louse." To us, though, as we study biology, the chapter entitled "Of the Schematisme or Texture of Cork, and of the Cells and Pores of some other frothy bodies" is of the greatest significance because its contains the world's first description of cells.

To make these observations, Hooke took a sharp pen-knife, sliced a piece of cork into a very thin wafer and then

"examining it very diligently with a Microscope, me thought I would perceive it to appear a little porous."

A second specimen followed the first with growing excitement.

"I could plainly perceive it to be all perforated and porous, much like a Honey-comb in these particulars..."

The 'pores' reminded him of the small cubicles occupied by monks in a monastery, so he took the Latin word "cella" meaning a little room and gave it to what he saw. Hence the word "cell" today.

"Next in that these pores, or cells, were not very deep, but consisted of a great many little Boxes, separated out of one continued long pore, by certain Diaphragms..."

All good scientists measure, and Hooke was no exception. He began to count.

"I told (meaning counted) several lines of these pores, and found that there were usually about threescore (60) of these Cells placed end-ways in the eighteenth part of an Inch in length...and therefore in a square Inch about a Million or 1166400, and in a Cubic Inch, about twelve hundred Million, or 1259712000, a thing almost incredible, did not our Microscope assure us of it by occular demonstrations..."

Some of his excitement comes over to us in these words, which he wrote in 1665, and the sensation is a familiar one to any good scientist today. A new, amazing world, the world of cells, had been laid open to investigation.

Hooke continued his investigations and found cells in the plants of his garden and nearby fields:

"Elder, Cany, Fennel, Carrots, Daucus, Bur-docks, Teasels, Fern, some Reeds and Vegetables."

He noted that the cells of green plants were filled with juices and were separate from each other, but he was not able to take his studies much further; his microscope was too crude and he knew nothing about specimen preparation. However the new science of cell biology was definitely under way.

By 1858, however, there had been a considerable improvement in both microscope and specimen preparation techniques. Many more people became interested in studying the world of cells, and gradually, as techniques became more refined and date more reliable, three universal properties of these 'units of life' emerged.

1. Cells had an incredible diversity in the shape, form and function. From building the stems of plants to filtering poisons in animal kidneys, cells were involved.

2. All animal and plant tissues were composed of groups of cells, or cell aggregates, and it was realized that the growth of a multicellular organism came about both from an increase in the total number of cells and an increase in the size of the existing cells. Cells were the 'building blocks' from which all other living things were constructed.

3. Every cell had its own independent existence, i.e. carried out its own respiration, metabolism, etc., and, most importantly, had the ability to reproduce. This was the idea two German biologists called Matthias Schleiden and Theodor Schwann who proposed the idea in 1838.

• all living things are assemblages of different kinds of cells.
• each of these cells maintains its own independent existence.
• new cells can arise only from the growth and reproduction of preexisting cells.