The aim of this booklet is to provide the microscopist with a basic explanation of the theory of the microscopy sufficient to enable him to understand the reasons behind accepted microscope techniques. It is felt that such an understanding will not only add to his interest in using the microscope, but will help him to work his way out of possible problems that may arise later on when detailed instructions, originally grasped, may have been forgotten. Where possible, mathematical formulae have been avoided, in favor of physical or pictorial explanation, as it is felt that such explanations are more easily grasped and better retained than explanations involving mathematics.