Nervous muscular tension, developed during a 30-min. period of reading, was appraised by changes in pressure as registered by the fingers of the left hand which rested naturally upon the knob of a concealed key. This criterion is regarded as an indirect means for appraising the complex psycho-physiological reactions to lighting, visual tasks and various conditions for seeing. A correlation between intensity of illumination and the pressure unknowingly exerted upon the key indicated that the average pressure decreased from 63 to 43 gm. as the intensity of illumination was increased from 1 to 100 footcandles. The probable errors of these values are of the order of 1%. By introducing a glare-source into the visual field of such a brightness as to produce definite discomfort, both glare and low levels of illumination produced similar indications of nervous muscular tension. These data do much to confirm the belief that intensities of the order of 1000 footcandles are desirable for the performance of critical visual tasks.