A report is presented investigating the fact that physical transparency and perceptual transparency are not necessarily coincident phenomena. A brief summation is given of previous research by H. von Helmholtz, E. Hering, W. Fuchs, K. Koffka, B. Tudor-Hart, G. Kanizsa, and W. Metzger. Certain mosaics of opaque colors and shapes will give rise to the impression of transparency. With these tools color fusion, color scission, and coefficients of reflectance are briefly considered in reference to a theoretical index of transparency. A simple theoretical model is developed which predicts those figural and chromatic conditions under which perceptual transparency will occur, derived with no empirical corrections or adaptations, and considering pure achromatic relationships. -- AATA