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The theory of image formation in a microscope proposed by Ernst Abbe changed the scientific approach to microscopy. Though the theory had many detractors, his new approach led to a technological revolution in the design and construction of high-quality microscopes. It paved the way for new discoveries in the fields of biology and medicine. Joaquín María de Castellarnau, was a contemporary connoisseur of Abbe’s ideas who decided to disseminate them in Spain through various publications and training courses. In his lectures, he used various devices for practical demonstrations that allowed some concepts of the new theory to be better understood. At the Museo Nacional de Ciencias Naturales in Madrid (MNCN–CSIC), one such original and unusual instrument designed by Abbe and used by Castellarnau has been preserved in perfect condition. Castellarnau used this instrument for various experiments that helped clarify the most complex points of Abbe’s theory. In this work, we explore how the context in which science developed in Spain favoured practical activities to demonstrate new scientific theories, such as Abbe’s in the early twentieth century.