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BJHS 38(2): 127–159, June 2005. f British Society for the History of Science doi:10.1017/S0007087405006709
The magic of the magic lantern (1660–1700):  on analogical demonstration and the visualization of the invisible - KOEN VERMEIR [Institute of Philosophy, University of Leuven, Kard. Mercierplein 2, B-3000 Leuven, Belgium]

I now shift to a different context, to the Paris and Versailles of the reign of Louis XIV. I focus on a description of the magic lantern in the work of Pierre le Lorrain, Abbe´ de Vallemont (1649–1721).

Vallemont was a theologian who worked as a private teacher and was employed in Paris by Pollart, a counsellor of the parlement, and in Versailles, by the Marquis de Dangeau, a favourite of Louis XIV. Vallemont read all the new scientific works, collected curiosities, explored the gardens of Versailles and often visited the collections of the king. He wrote popular works on physics, gardening and numismatics and was always engaged in contemporary disputes.

He mentioned the magic lantern in his popular book La Physique occulte (1693), where it appears in the context of a physical discussion of the nature of light.

Vallemont is a (pseudo-)Cartesian atomist, and he describes light as rays of subtle particles. This makes it surprising that light particles are not dispersed by wind, and that different light rays can pass together through the same point without affecting each other. The rays do not mingle and their forms and colours are preserved. In order to demonstrate this, Vallemont invokes the magic lantern and sets out how it is constructed, thereby explaining that the rays or traces of particles become one and penetrate each other in the tube of the lantern because of the lenses. Yet the rays keep their direction and draw ‘the phantom of the object with all its colours ’ on the screen.

The use of instruments to demonstrate physical principles was already common in the Cartesian circles of Paris. From the 1660s, Jacques Rohault staged the wonders of nature in an elaborate play. He demonstrated numerous phenomena with his wide assortment of instruments and provided them with Cartesian explanations.

Vallemont referred to the authority of Rohault’s demonstration lectures and he used the latest curiosity in a similar vein – to demonstrate a physical phenomenon in a Cartesian setting.

But Vallemont did more, and this becomes clear if we recognize that his demonstration of the behaviour of light rays is a key moment in an argument on the physical explanation of dowsing.

La Physique occulte was written in the context of a debate on the divining rod, evoked by a spectacular case of dowsing at the time. [A peasant had discovered murderers with a divining rod; for the episode and performed experiments see M. R. Lynn, ‘Divining the Enlightenment: public opinion and popular science in old regime France’, Isis (2001), 92, 34–54. Other cases of dowsing were well attested by reputable witnesses, experiments were performed and even Boyle had shown interest.]

Vallemont denied any demonic intervention in dowsing and set out to explain the phenomenon by means of very subtle particles. Water sources, gold and even murderers could be found by the specific kind of vapours they exhaled, and the divining rod worked ‘like a microscope’, enlarging the effect of those vapours on man. When tracing murderers, the diviner followed a stable track that remained unaltered for several hours.

Vallemont had to demonstrate that the subtle particles were not mixed and blown away by the wind. He argued that those particles were so subtle that they did not interact with the coarser air particles, and that the whole world was filled with subtle particles which did not modify each other. This explanation would have been very dubious if Vallemont had not been able to demonstrate that there existed particles with exactly such a behaviour.

It is precisely this that the magic lantern had to demonstrate: light particles behaved like the occult particles Vallemont envisioned. The wind did not blow away the subtle transpirations exhaled by the murderers, just as the wind did not blow away the light rays and the luminous image projected by a magic lantern. Even if different people came together at one place and separated again, their specific transpirations would not mix and lose their distinctiveness. That is why the divining rod would still be able to trace these people separately, just as the different colours of the lantern image are not modified even if the light rays go through one single point somewhere in the tube of the magic lantern. This makes it clear that the magic lantern was not only used as a metaphor for the eye and as a demonstration of a physical principle. It was also an ‘analogical demonstration’ of the properties of occult or invisible particles.

Vallemont proved that his explanation of dowsing was probable by arguing that there were similar phenomena which really could be demonstrated. As in Kircher’s case, the magic lantern serves to make the invisible visible.

The world is full of occult (imperceptible or hidden) powers such as magnetism, electricity or even light, and only the effects of these occult powers are visible.

Cartesians explained these phenomena by introducing subtle particles with certain forms. To convince the reader, they often invoked analogical models, to make it easier to conceive the hidden workings of nature. These analogies were relatively easy to construct and appealed to the public imagination. Cartesianism became a game in itself when it was adapted by court and salon culture. It was a game for the diversion of the gentry with sophisticated and alluring analogies and explanations of natural phenomena.

Vallemont combined this trend for analogical reasoning with the fashion for demonstration lectures in a specific way. By demonstrating a known principle of light and by using an analogy between light and subtle transpirations, he achieved something very different from other demonstrators and Cartesians.

An analogical demonstration is used in a similar way as an experiment, to create new knowledge. In experimental science, heuristic analogies are usually tested in experiments upon the real thing. If proved valid, the analogy might be used in demonstrations to elucidate the tested and known principle. Vallemont, however, employs an analogical demonstration, based on an unquestioned analogy, to generate new knowledge of the interactions of invisible and unknown vapours. By introducing the magic lantern at a crucial phase of his argument, Vallemont succeeded in visualizing his theory in a convincing way, drawing upon the growing authority of experimental philosophy and on the fashion for demonstration lectures. By means of an analogical demonstration he in fact demonstrates the indemonstrable. By analogy he transforms his occult particles into light particles and by means of the magic lantern he makes those invisible light particles visible. That is, in the luminous projected image one can clearly see that the colours are not mixed up or blown away.