1. In Part I the principles of reflexion densitometry are discussed in relation to the measurements of visual pigments in man.2. In Part II the new Florida densitometer is described in some detail, and records are shown of the instrument's performance with pigment in equilibrium or changed by bleaching.3. With an artificial eye the fluctuations are of the order to be expected from quantum noise and limit the precision of one measurement to 0.0035 of ;double density' which corresponds to some 2% of total pigment.4. Part III treats of the reliability and significance of the measurements in living man. It needs ten repeated measurements to attain the reliability of 1 with the artificial eye.5. The measuring light used bleaches cone pigments by 0.4% in the 6 sec required to make a measurement.6. The interpretation of the measurements is briefly discussed.