Over the last dozen years there has been a great development in the study of matter on the micro-scale by means of the light microscope, which has coincided with the rapid growth of auto mation and the use of computers. The manufacturers have responded to this challenge, and there is now available a large choice of modules for use with the microscope, both on the biological and non-biological side. This book is on microscope photometry. Photometry on the macro-scale can be applied to features of not less than about one millimetre in size. Below this lies the realm of microscope photometry which will be discussed both in theory and in practice. As a general rule no discussion of technical design of equipment is included, as such information is available in the handbooks of the particular manufacturers of equipment. It attempts to cover all the uses of the optical microscope photometer with its auxiliary equipment, and the first chapter describes the topics covered. In using the word' light' we apply its meaning to extend from UV to near IR. No attempt has been made to list all the works consulted. For each subject treated in this book a list is given of books and papers considered to be of key importance; in these will be found more detailed bibliographies.