It has long been the opinion of many textile technolo gists that some precise and easily duplicated means of determining the index of refraction would be of paramount value in investigating the structure of textile fibres as well as assisting in their positive identification. Such other phenomena as the effect of moisture content, the effect of strain or the so-called "memory" of plastics (such as silk, wool, rayons, etc.), the identification of faulty yarn due to excessive tension or other orientation of structure,† the effect of mercerization, the correlation of index values with the quality of wool or the grade of cotton—all offer fields of study in fibre research provided that a precise means of refractive index determination is possible. It was with these problems in mind that the present work was begun on a more precise and convenient method of measuring the refractive indices of individual textile fibres.