The seventeenth century brought astronomical changes to the world of science and optics, literally and figuratively. The invention of the telescope and microscope in the 1590s triggered enormous interest in exploring previously unobservable realms. The observations made from those explorations would transform human understanding of the world and the universe.

In 1608, Hans Lippershey made improvements to the original design of the telescope and reported them to Galileo. Within a year, Galileo had built his own telescope and discovered the moons of Jupiter, one of the observations that validated the Copernican theory. However, the Church at that time was not willing to accept his findings and he was forced to publicly recant his support of the Copernican worldview.

In the second half of the century, Robert Hooke and Antonie van Leeuwenhoek published books with some of the observations they had made through their microscopes. The books included illustrations as well as descriptions, captivating readers with previously unknown details of everyday objects and the previously unseen world of microbes.

With better tools and a greater tolerance of observation and experimentation, scientists began to broaden their understanding of the natural world. In 1604, Johannes Kepler published a major work on the nature of light and optics, expanding on Witelo's Perspectiva, the most important work that had been produced in the medieval era. In his work, Kepler explained in greater detail how vision works; light enters the eye, then is refracted and focused through the lens onto the retina. With this insight, he was the first to explain farsightedness and nearsightedness and why corrective lenses work. Realizing that the farther light travels from a source, the dimmer it becomes, Kepler developed and introduced the inverse square law that describes the mathematical relationship between light intensity and distance.

Many other discoveries about the nature of light were made during this time: several scientists defined the geometry of light refraction and reflection more precisely, Francesco Grimaldi theorized that light has a wave nature, Erasmus Bartholin discovered double refraction in certain crystals, Isaac Newton discovered that white light can be separated into different colors, and Ole Roemer concluded from his measurements that light does not travel instantaneously, but travels at a finite speed.

Throughout the previous centuries, the Church had been intricately involved with scientific studies, but towards the end of this century, scientists began to separate themselves from the Church hierarchy. Scientists developed their own organizations to discuss and evaluate their work and the sciences began to function as organized disciplines. In England, a number of small discussion groups merged in 1660 to form the Royal Society of London for the Promotion of Natural Knowledge. In France, the Paris Academy of Science formed in 1666. Organizations like these would greatly influence the development of the sciences in Europe over the next two hundred years.

By the time Isaac Newton published his Principia in 1687, the universe was no longer regarded as changeless and perfect and the Earth did not figure as its center. The Copernican theory had been universally adopted in Europe, updated with newly acquired knowledge.

1600 - 1699 1604 Johannes Kepler (Germany) publishes a major work on optics, Ad Vitellionem Paralipomena, Quibus Astronomiae Pars Optica Traditur (Supplement to Witelo, in which is Expounded the Optical Part of Astronomy). In it he states that the intensity of light from a source varies inversely with the square of the distance from the source; he describes vision as a result of images on the retina created by the lens in the eye; he correctly identifies the causes of farsightedness and nearsightedness. 1608 Dutch lens maker Hans Lippershey (also called Hans Lippersheim) builds a telescope consisting of a converging objective lens and a diverging eye lens. He reports his invention to Galileo. 1609 Galileo Galilei (Italy) builds a telescope modeled from Lippershey's telescope and uses it for astronomical observations. Later in the year, he draws pictures of the moon's phases as seen through the telescope and in January 1610 he discovers that Jupiter has four moons. 1610-1611 Three observers--Galileo, Christopher Scheiner, and Johann Fabricius (son of David Fabricius)--discover sunspots using the newly invented telescope. Galileo risks blindness by looking at the sun directly through his telescope. The others use safer methods, such as a camera obscura, to observe the sun indirectly. 1611 Johannes Kepler (Germany) publishes a treatise, Dioptrice, in which he proposes a new design for the telescope using two convex lenses. This will eventually become the classic design for an astronomical telescope. 1613 François d'Aguilon (Belgium) publishes Opticorum Libri Sex (Six Books of Optics), which adds a number of original insights and contributions to the field of geometrical optics. 1613 Perhaps following Kepler's recommendation, Christopher Scheiner (Germany) perfects the design of the refracting telescope, using two convex lenses instead of one convex and one concave (as built by Galileo). 1614 Italian chemist Angelo Sala publishes a pamphlet about his experiment with silver salts. He notes that when powdered silver nitrate is exposed to the sun "it turns black as ink." 1616 Nicolas Zucchi (Italy) constructs an apparatus in which an ocular lens is used to observe the image produced by reflection from a concave metal mirror. This is one of the earliest reflecting telescopes, in which the enlargement is obtained by the interaction of mirrors and lenses. 1619 Inventor Cornelius Drebbel (born in Holland but residing in England) develops a machine for grinding lenses and constructs a compound microscope and a camera obscura with a lens in the aperture. 1621 Physicist Willebrord Snell (Holland) discovers the law of refraction and determines that transparent materials have different indices of refraction depending upon their composition. He does not publish his discovery, however, and it remains unknown until 1703 when it is published by Christiaan Huygens. 1633 Galileo is forced by the Inquisition to recant his support of the Copernican theory that the Earth and other planets revolve around the sun. 1637 In appendices to his book Discourse on the Method and Essays, René Descartes (France) explains rainbows and publishes his discoveries about the laws of reflection and refraction. He discovers Snell's law of refraction independently, but is the first to publish it. 1638 Dutch astronomer John Phocylides Holwarda establishes that Mira Ceti (also called Omicron Ceti) is a variable star, not a nova, and exhibits a 332-day cycle in its luminosity. 1647 Bonaventura Cavalieri (Italy) describes the relationship between the radius of curvature for the surface of a thin lens and its focal length. 1658 French mathematician Pierre de Fermat defines his principle of "least time," which suggests that a ray of light will travel in a path that will allow it to get to its destination in the shortest amount of time. His principle agrees with Snell's law of refraction. 1660 Italian physiologist Marcello Malpighi first uses the microscope to examine blood capillaries. Several years later (in the late 1660s and 1670s) he studies the stratum malpighii in the skin, and the malpighian corpuscles in the liver and spleen. He also employs the microscope to study the development of the chicken embryo. 1663 James Gregory, a Scottish mathematician and astronomer, describes the first practical reflecting telescope in his work The Advance of Optics. He also introduces the estimation of stellar distances by photometric methods. 1664 Robert Hooke (England) is the first to build a Gregorian reflecting telescope. He uses it to discover a new star in the constellation Orion and make observations of Jupiter and Mars. Hooke also is the first to discover plant cells in fossil wood using his compound microscope. 1665 Two years after his death, Francesco Maria Grimaldi's book Physicomathesis de lumine, coloribus, et iride, aliisque annexis is published detailing his observations of the diffraction of white light. In his book, the Italian physicist concludes that light is a liquid capable of wave-like motion; one of the earliest indications that light behaves like a wave. 1665 Robert Hooke publishes Micrographia (Small Drawings), his studies and illustrations of objects and tiny organisms viewed through his microscope, including a flea and a louse. In the same year, Dutch microscopist and naturalist Jan Swammerdam observes erythrocytes and the two-cell cleavage stage of frog eggs with his simple microscope. 1666 Isaac Newton (England) finds that white light separates into different colors when it passes through a prism. 1668 Frustrated by refracting (Galilean) telescopes that alter the color of astronomical objects (chromatic aberration), Isaac Newton invents and builds a reflecting telescope of his own design, but based on the suggestions of James Gregory. 1669 Erasmus Bartholin (Denmark) discovers double refraction when he sees an image separate into two images when viewed through a piece of Iceland Spar crystal. 1672 In his first letter published in the Royal Society's Philosophical Transactions, Isaac Newton reports about his prism experiment, concluding that white light is composed of different colors that are refracted at different angles by a prism. 1676 Based on his observations of the time elapsed between eclipses of Jupiter's moons by Jupiter, Ole Roemer (Denmark) concludes that the speed of light is finite and estimates it to be about 140,000 miles (225,000 kilometers) per second. 1678 Christiaan Huygens sends a letter to the Academie des Science in Paris presenting his wave theory of light, which will be published later in his Traite de Lumiere in 1690. 1683 Antonie van Leeuwenhoek (Holland) publishes his first letter in the Royal Society's Philosophical Transactions, with descriptive illustrations, about his observations of microscopic "animalcules". He builds his own microscopes, with magnifications ranging from 50 to 300 times and is the first to make descriptive drawings of protozoa, bacteria, spermatozoa, and red blood cells. 1687 Isaac Newton publishes Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), launching the culmination of the Scientific Revolution.

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