Christiani Hugenii Zelemii, dum viveret, toparchae opuscula postuma, quae continent dioptricam. Commentarios de vitris figurandis. Dissertationem de corona & parheliis. Tractatum de motu. De vi centrifuga. Descriptionem automati planetarii

but many things that were neither complete enough nor arranged in a convenient order, as they were things upon which the Author had deferred putting the final hand, being distracted by other meditations that had arisen, as often happens. We have judged, however, that we would neither deviate from the Author's intention nor offend the interests of the literary Republic if, in addition to those earlier works specifically prescribed to us, we appended to the Dioptrics and the Treatise on Forming and Polishing Lenses a Dissertation on Parhelia sun dogs; to the Treatise on Motion by Percussion, another On Centrifugal Force; and finally, a Description of the Planetary Automaton, by which our Author imitated the motions of the celestial planets on a flat plane. Besides these, we indeed found some other things begun by the Author, but by no means brought to such a state that, in our judgment, they could see the light of day. That these are published somewhat late is due partly to the fact that these writings came into our hands late, and partly because the distance between the places where we live brought no small delay, as it was necessary for each of us to review them separately; nor was it possible, if any matter required deliberation, to confer in person and decide on something certain, but rather to conduct everything by letters—which, for many reasons that anyone can easily perceive, entails much labor and a great waste of time. To these, finally, are added our private affairs, which have often distracted us from completing these tasks.

In the first place, therefore, one can see the Dioptrics, written many years ago, just like almost all the others. From those things that the Illustrious Huygens had annotated here and there, it appeared to us that he had intended to add to this Dioptrics the Reflecting Telescope of Newton original: "Speculare telescopium Newtoni" and those things that concern its aperture; the doctrine of Parhelia; the description and demonstration of a Level original: "Libellæ"; and a Compendious Astroscopy original: "Astroscopiam compendiariam"; of which, however, we have added nothing here except the Treatise on Parhelia, since the description and demonstration of the level...