Cartesian corpuscular models and their potential impact on microscopy
Delphine Bellis Université Paris – Sorbonne ( Paris IV)

Far from the picture describing him as a novelist-physicist, lost in his speculative theories, Descartes had always shown a real interest in practising experiments of all kinds. As a result, we should not find it surprising that he devoted much of his time to optics: theoretical optics as well as practical optics (designing lenses, having them ground, discussing with Ferrier or Constantin Huygens the different results obtained by processes of trial and error, trying to find technical solutions to get more precise lenses...).

The interest Descartes displayed for optical instruments should not be reduced to a wish to improve people’s eyesight for everyday life. Nor should it be limited, as might be infered from his Dioptrique, only to an ingenious device revealing the fruitfulness of his method. On the contrary, we would like to argue that Descartes’ investigations into optics are strongly connected to the usefulness of instrumentation for the knowledge of natural phenomena. To sustain our claim, we recall Descartes’s position expressed at the beginning and at the end of the Dioptrique:

“en sorte que portant notre vue beaucoup plus loin que n’avait coutume d’aller l’imagination de nos pères, [les lunettes] semblent nous avoir ouvert le chemin, pour parvenir à une connaissance de la Nature beaucoup plus grande qu’ils ne l’ont eue”.

“je les juge toutefois beaucoup plus utiles [que les lunettes astronomiques], à cause qu’on pourra voir par leur moyen les divers mélanges et arrangements des petites parties dont les animaux et les plantes, et peut-être aussi les autres corps qui nous environnent, sont composés, et de là tirer beaucoup d’avantage pour venir à la connaissance de leur nature…”.

However important these two remarks should appear for anyone who wishes to understand the real links between theory and experiment in Descartes’ physics, they are rarely ever noticed. Nevertheless, our purpose will precisely consist in showing that Descartes had dreamt of an increase of vision through instrumentation that could lead to sustain and even complete his theoretical corpuscularism. A few years later, Henry Power, in his Experimental Philosophy, should underline the connection between Descartes’ corpuscularism and later observations of the microscopical world:

“With our Artificial eyes we might hope, ere long, to see the Magnetical Effluviums of the Loadstone, the Solary Atoms of Light (or globuli aetheri of the renowned Des-cartes), the springy Particles of Air, the constant and tumultuary motion of the Atoms of all fluid Bodies, and those infinite, insensible Corpuscles which daily produce those prodigious (though common) effects amongst us”.

Whereas ancient atomism had never given rise to precise experimental research, Cartesian corpuscularism can be analyzed as the theoretical position that would lead Descartes to develop his interest for instrumental observation of nature. But why Descartes himself never accomplished the dream of making corpuscles visible is what we shall pay attention to. To say it in a few words, this renouncement may be linked to the limited performances that could be reached by optical devices at the time, but also to the conceptual nature of Descartes’ corpuscularism as a causal model. However, his entire early physics bears the signs of such an ideal. We will try to identify such signs in precise procedures of theorizing in physics (in Le Monde, Les Météores,or La Dioptrique) and therefore determine the role experiments should endeavour to play in his own physics: what could microscopy reveal on bodies and how could Descartes think to integrate these data into his theories? Could we ever think that these data might have been quantified? We will also find in the works and experimental procedures of some of his followers (such as Power, Hooke, or Boyle) the impact of such a way of considering the relations between corpuscularism and microscopical experiments. These relations, as will be seen, are never simple or univocal, but they reveal the problematic link between a corpuscularian model which could never be deduced as such from metaphysical principles and experiments about which Descartes says they are a necessary component of his physics to test his hypothetico-deductive models.

In this view, we will however be able to understand that Descartes’ thought does not only consider vision, as is often depicted, as an illusion unable to give us access to the true nature of things. But it is only in the context of a reflexion that evaluates the mechanisms of seeing that optical instrumentation should find its place in stimulating physical research.

Our interpretation therefore departs from that exposed by Neil M. Ribe in “Cartesian Optics and the Mastery of Nature”, in Isis, 1997, vol. 88 (1), p. 42-61.

Dioptrique, discours premier, in Œuvres complètes de Descartes par Charles Adam et Paul Tannery, Paris 1897-1909 ; revue et augmentée, Vrin, 1996. (cited AT), t. VI, p. 81.

Dioptrique, discours dixième, AT VI, 226. Descartes’ concern is here about what we would call “microscopes”.

Experimental Philosophy , preface, London, 1664.

This possibility can be thought of from Descartes’ reflection on sensible minima.

Cf. Discours de la méthode, VI, AT VI, 64-65: “Mais il faut aussi que j’avoue que la puissance de la Nature est si ample et si vaste, et que ces principes sont si simples et si généraux, que je ne remarque quasi plus aucun effet particulier, que d’abord je ne connaisse qu’il peut en être déduit en plusieurs diverses façons, et que ma plus grande difficulté est d’ordinaire de trouver en laquelle de ces façons il en dépend. Car à cela je ne sais point d’autre expédient, que de chercher derechef quelques expériences, qui soient telles, que leur événement ne soit pas le même, si c’est en l’une de ces façons qu’on doit l’expliquer, que si c’est en l’autre.”