CHAPTER ONE

THE MICROSCOPE AND THE CELL

The cell theory

The idea that all living creatures are made of cells has changed more than anything else our concept of life, and is still the fundament of modern biology. This great generalization was made possible by the invention of the microscope, but did not come suddenly. It has been instead the culmination of a collective research which lasted more than two hundred years, and in order to understand it we must be aware of the main problems that had to be solved.
Let us start with the microscope. Why do we need it? Why can’t we see the cells with the naked eye? The answer is that the eye’s retina itself is made of cells. Two objects can be seen apart only if their light rays fall on different cells of the retina, because if they strike the same cell the brain receives only one signal. More precisely, the brain can tell two objects apart only when their images on the retina have a distance of at least 150 microns (thousandths of a millimetre). The cells have average dimensions (10 microns) far smaller than that limit, and, even if an organism is stared at a very close distance, their images overlap and they remain invisible. It is therefore necessary to enlarge those images in order to increase their distance on the retina, and that is where the microscope comes in.
Enlargements of 5 or 10 times can be obtained with a single lens (the so-called simple microscope) but are not enough for seeing the cells. Substantially greater enlargements require a two-lens system (a compound-microscope) and the turning point came in fact with the invention of that instrument. The first two-lens optical systems were the telescopes, and the idea of a compound microscope came essentially from them.