The optical lantern projector was first introduced as a teaching aid into schools all over the world at the turn of the twentieth century. Because slides were expensive, special slide lending services played an important role in supplying schools with images they could project. Existing studies on the use of this new medium in education have hinted at the influential position of these slide libraries but have never analysed them as a phenomenon in itself. Based on a wide variety of archival and published sources, this contribution charts the slide lending service "landscape" in Belgium. It demonstrates how these slide libraries were framed as weapons in the intense conflict between Catholics and free-thinking Liberals that occupied Belgium for most of the nineteenth and early twentieth centuries, and in which schooling was a main point at issue. By analysing the slide collections of these organisations by means of their catalogues, this article shows to what extent ideology influenced the topics that were represented in the materials they provided. It also shows that the opposing sides reached common ground when it came to their home country. Since the international trade did not offer the types of slides they wanted to show to their pupils, teachers working in slide lending services often created their own slide series. In their slide sets on Belgium, both non-confessional and Catholic lending services represented their relatively young country as one geographical entity of which its inhabitants could be proud.