apparatus for measuring the secretions of sebum of a skin sample of a living subject comprises a frosted glass plate spring urged to project from a casing . when pressed against the forehead of a living subject the frosted plate retracts yieldably to collect sebum under reproducible conditions . a reference scale inside the casing of the apparatus is illuminated from a known light source and is observed from outside by looking through the greasy plate . the sebum secretion is measured by changing the position of the scale , or changing the line thickness of a reference mark on the scale until a sharp , fine image of the reference mark can be seen .