Descartes: A Biography

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 Descartes: A Biography

his hands are about two feet apart, and he turns his two sticks inward so

that they both reach the same object in his vicinity. If he estimates the

angle by which he bends his sticks from a parallel position, it is an easy

mathematical problem, at least for Descartes, to calculate the distance of

the invisible object.This suggests that there is a form of spontaneous

triangulation involved in our estimation of the distance of objects that we

can see, because we turn our eyes from their normal parallel position to

focus on objects of perception that are more or less close to us.

The remainder of the text discusses the ways in which lenses of dif-

ferent shapes are suitable for various optical instruments, and especially

the merits of hyperbolical and elliptical lenses, which Descartes claims are

‘preferable to any others that can be imagined’ (vi.). It is easy to appre-

ciate from theDioptrics,inretrospect, the urgency of Descartes’ invitation

to Ferrier to follow him to the United Provinces when he first arrived in

, and the frequency with which he had written to Huygens, in the

s,requesting assistance in finding a lens grinder who could produce a

uniformly ground hyperbolic lens. His theory of vision and his research on

dioptrics could be tested only by using well-ground lenses. Accordingly,

the final discourse of theDioptricsreproduces a design for a good lathe for

lens grinding, similar to that described in lengthy correspondence with

Ferrier in October and November.Descartes still had not yet found

a sufficiently skilled lens grinder. But, in the meantime, he was publishing

a theory that could be confirmed or otherwise by experiment.

TheMeteorsis, like theDioptrics,asomewhat eclectic collection of

explanations of meteorological phenomena, most of which had been dis-

cussed by Descartes in correspondence during the previous nine years. In

addition to discussing the disparate phenomena mentioned, it also had as a

centerpiece a prominent new discovery, namely, the Cartesian explanation

of the rainbow. It is clear from the outset–apointreiterated in the con-

cluding sentence of the book – that Descartes’ objective was to demystify

the apparently strange phenomena that occur in the space between the

Earth and the heavens and that had been interpreted by others as omens

or prophetic messages. ‘That makes me hope that, if I explain the nature

of clouds here[,]...it would be easy to believe that it is possible, in the

same way, to find the causes of everything that is most admirable above

the earth’ (vi.).The explanations that are suggested are contrasted

with the ‘superstition and ignorance’ (vi.) that compromises even the

very description of many meteorological events.