Method of preparing a multi-layer reflection reducing coating

Claim 1

THE METHOD OF PREPARING A MULTI-LAYER COATING AT LEAST ONE LAYER OF WHICH IS AN OPTICALLY UNIFORM THIN LAYER CONSISTING OF AT LEAST TWO SUBSTANCES HAVING DIFFERENT REFRACTIVE INDICES, WHICH COMPRISES FORMING A MIXTURE OF THORIUM FLORIDE AND ZINC SULFIDE, COMPACTING SAID ADMIXTURE INTO PELLET FORM UNDER VACUUM, HEATING THE PELLETIZED ADMIXTURE TO A TEMPERATURE AT WHICH BOTH SUBSTANCES EVAPORATE, DEPOSITING THE RESULTING VAPORS ON A RECEIVING SURFACE TO FORM THEREON A COATING LAYER OF A MIXTURE OF THORIUM FLUORIDE AND ZINC SULFIDE OF A THICKNESS OF ABOUT 0.24$, SUPERIMPOSING OVER SAID FIRST LAYER A CLOSELY ADHERENT LAYER OF ZINC SULFIDE FREE OF THORIUM FLUORIDE OF A THICKNESS OF ABOUT 0.47$ BY VAPORIZATION, AND SUPERIMPOSING OVER SAID SECOND LAYER A CLOSELY ADHERENT LAYER OF MAGNESIUM FLUORIDE OF A THICKNESS OF 0.25$ BY VAPORIZATION TO PRODUCE A TRIPLE LAYER COATING ON SAID SURFACE.