In considering the examination of transparent mineral plates, it has already been seen that there are three conditions of the microscope, all of which must be used in turn if we are to acquire a satisfactory amount of information in regard to the minerals in thin section. Even after this examination, it is not always possible to say with certainly that a particular mineral has been satisfactorily identified. The need for some further information is sometimes keenly felt. Convergent polarized light often supplies us with a means of acquiring the required information.