A definition of refractive index (RI) is followed by a discussion of its relevance to glass repair. The four available methods for RI measurement are compared. The authors examined the RI of a variety of commercial adhesives and of typical historical and artistic glasses. These are compared, with Glass Standards, in Table 2. The authors also discuss the possibility of marginally modifying the RI of particular commercial adhesives by the following methods: using different hardeners with one resin, marginally changing the proportion of hardener, adding plasticizers or diluents. It is noted that the tendency of a resin to yellow is of less importance than the ability of the RI to match the glass undergoing repair. See 21-1334 for reference to book. -- AATA