Giambattista Della Porta: A Magician or an Optician?

Research output: Chapter in Book/Report/Conference proceedingChapterpeer-review


In Bk. 17, Ch. 4 of Magia Naturalis (1589) Giambattista Della Porta (ca. 1535–1615) reported his experiments on concave spherical mirrors arranged in various setups. Della Porta identified two critical points: (1) the point of inversion (punctum inversionis) in reference to the place where the magnified image is turned upside down and seen blurred, and (2) the point of burning (punctum incensionis) in reference to the place where the reflected rays concentrate and ignite fire. Opticians and practitioners of the time distinguished between the two points but considered them to occupy the same spatial location. Della Porta inferred from his studies of concave spherical mirrors that the position of the point of inversion and that of the point of burning occupy different spatial locations. He associated the point of inversion with a locus where the image is seen magnified, turned upside down and blurred—a matter of visual perception. He defined the point of burning as a physical, optical position associated with a geometrical point in which the converging rays ignite fire. Consequently, throughout Bk. 17, Della Porta discarded the point of inversion from his optical nomenclature and referred only to the point of burning, the real—so to speak—optical point. In so doing, Della Porta contributed fundamentally towards the technological management of sets of optical elements. In this paper we follow the experimental practice of Della Porta as presented by the optical demonstrations in Bk. 17, Ch. 4. We discuss the theoretical principles Della Porta developed to clarify whether his claim concerning concave spherical mirror is hypothetical or was it based on an inference from experience. We offer novel insights into the development of the theory of reflection in concave spherical mirrors as it was pursued by Della Porta. He eliminated perceptual considerations from his optics and considered only geometrical-physical aspects. This approach was most useful in the development of the telescope where the critical aspect is not perception but rather ratio of spatial angles.

Original languageEnglish
Title of host publicationArchimedes
EditorsA. Borrelli, G. Hon, Y. Zik
PublisherSpringer Nature
Number of pages17
StatePublished - 2017

Publication series

ISSN (Print)1385-0180
ISSN (Electronic)2215-0064

Bibliographical note

Funding Information:
Acknowledgments We gratefully acknowledge the helpful correspondence and ensuing discussions with A. Mark Smith and his critical comments related to the translation of Della Porta’s De Refractione. This research is supported by the Israel Science Foundation (Grant No. 67/09).

Publisher Copyright:
© 2017, Springer International Publishing AG.


  • Concave Mirror
  • Geometrical Point
  • Optical Element
  • Optical Point
  • Optical Simulation

ASJC Scopus subject areas

  • History
  • History and Philosophy of Science
  • Philosophy


Dive into the research topics of 'Giambattista Della Porta: A Magician or an Optician?'. Together they form a unique fingerprint.

Cite this