By the mid-1800s, the wave theory of light had been firmly established and measurements of the speed of light were more precise. However, an understanding of the nature of light, particularly how it propagates through the vast reaches of space, still eluded a satisfactory explanation. Curiously, it was another area of physics that would revolutionize the theory of light, the study of electricity and magnetism.

Michael Faraday, an English chemist and physicist, was the first to use magnetic fields to produce an electric current, which led him to theorize that magnetism and electricity are aspects of one force. He also speculated that light may be yet another aspect of this force. James Clerk Maxwell, a Scottish mathematician and physicist, followed up on Faraday's speculations and provided a unified theory of electromagnetism in 1865 which predicted the existence of a new phenomenon: electromagnetic waves.

Maxwell predicted that these new waves would travel at approximately the same speed as the speed of light. Not believing this to be a coincidence, he predicted that experiments would show light to be a form of electromagnetic wave. The experimental evidence for Maxwell's predictions would come later in the century.

Another development in the study of light during this period was the introduction of spectroscopy, the study of light spectra. By mid-century, scientists had learned that the spectrum from a given light source contained a wealth of information about its chemical composition. Experiments by German physicist Gustav Kirchhoff demonstrated that each element, when heated to incandescence, gives off a characteristic color of light. When the light is separated into its constituent wavelengths by a prism, each element displays a unique pattern. This made it possible to use spectroscopic analysis to identify the chemical composition of substances.

In 1861, Kirchhoff and another physicist, Robert Bunsen, showed that gases of elements would absorb specific wavelengths of light. This explained the mysterious dark lines (Fraunhofer lines) in the sun's spectrum and meant that it was possible to identify the chemical composition of distant objects like the sun and other stars.

1839 saw the genesis of photography on two fronts. French painter Louis Daguerre improved on the techniques developed by Joseph Niepce and produced photographs, or "daguerreotypes," on silver or silver-coated copper plates. While the quality was extraordinary, the exposure times were long and only one copy of each exposure could be made. At the same time, William Talbot, an English chemist, invented another photographic process that used paper treated with light-sensitive material. The result was a photographic negative, which could then be used to create many print copies--a tremendous advantage. The picture quality wasn't as good as the daguerreotype and the exposure time was long, but Talbot improved on his technique in 1840, dramatically lowering the exposure time. Although daguerreotypes were used for a number of years, eventually the photo negative and print copy technique established itself as the main method of photography.

Throughout the mid-1800s, optics for microscopes and telescopes continued to improve in quality, as did the number and availability of these instruments. While many astronomers ground their own lenses and built their own telescopes, quality microscopes were produced commercially. By the 1850s, a wide selection of microscopes was being produced in Britain and Europe. Commercial production didn't begin in the United States until the 1840s, but the new industry quickly made good quality microscopes more affordable and available for American scientists. The most significant innovation for microscopy during this time was the oil immersion technique. Introduced by Giovanni Battista Amici in 1840, it increased resolution by immersing the object of study in a medium with a higher refractive index than air.

1834 - 1866 1838 Sir Charles Wheatstone (England) describes the theory of stereoscopic vision and his invention of the stereoscope to the Royal Society. 1839 Niepce's crude photographic technique (1822) is refined by his colleague, a painter named Louis-Jacques-Mandé Daguerre (France). Daguerre produces excellent pictures, or "daguerreotypes," using silver-coated copper treated with a better quality light-sensitive chemical. 1839 William Talbot (England) invents a photographic process using paper coated with light-sensitive chemicals. Exposure through a camera obscura creates a photographic negative from which many prints can be produced. Later that year, Talbot accidentally discovers the latent image phenomenon, the invisible configuration of silver halide crystals on a piece of film. This dramatically reduces exposure time from one hour to one to three minutes. Talbot names the improved photographic process calotype. 1840 John Herschel (England) discovers Fraunhofer lines in the infrared region, the spectral region his father, William, discovered 40 years earlier. 1840 Pierre Louis Guinaud (Switzerland) develops a method for producing uniform optical glass. 1840 Giovanni Battista Amici (Italy) introduces the oil-immersion technique to microscopy. This minimizes light aberrations by immersing the objective in an oil film covering the specimen being studied. 1842 Alexandre Edmond Becquerel (France) photographs the sun's spectrum using a slit, a flint glass prism, and a lens to focus the image onto a daguerreotype plate. The photograph reveals the Fraunhofer lines of the solar spectrum, from the red region through the ultraviolet. 1842 Christian Johann Doppler (Austria) proposes that the observed frequency of light and sound waves depends on how fast the source and observer are moving relative to each other. 1845 Michael Faraday (England) defines an effect that is observed when the plane of polarized light that is passed through glass in a magnetic field is rotated. This is eventually called the Faraday effect. 1845 William Rosse, third Earl of Rosse, completes building the Birr Castle 72-inch optical reflecting telescope in Parsonstown, Ireland. 1846 In a public lecture, physicist/chemist Michael Faraday (England), who established that electricity and magnetism are two aspects of the same force, speculates that light may be yet another aspect of this force. 1847 Maria Mitchell (USA) is the first to discover a "telescopic" comet, a comet that can only be seen with a telescope but not with the naked eye. 1849 Armand-Hippolyte-Louis Fizeau (France) is the first to experimentally determine a reasonably accurate value for the speed of light. His experiment utilizes a rotating cogwheel and a fixed mirror placed several miles away to measure how fast light travels from the source light to the mirror and back. 1849 Henry Clifton Sorby (England) uses the polarizing microscope to examine rock sections for the first time. 1849 Sir David Brewster (Scotland) develops a model of the stereoscope, a viewer for stereoscopic prints that will become a popular item in Victorian drawing rooms. 1850 Jean-Bernard-Leon Foucault (France) measures the speed of light at 298,000 kilometers per second using the rotating mirror method. Within a year he uses the same method and finds that the speed of light is different in still water then the speed of light in air. 1854 George Gabriel Stokes (Britain) theorizes an explanation of the Fraunhofer lines in the solar spectrum. Stokes suggests that these are caused by atoms in the outer layers of the sun absorbing certain wavelengths, but does not develop or publish his theory. 1855 Giovanni Amici demonstrates water-immersion lenses for the microscope. 1855 David Alter (USA) describes the spectrum of hydrogen and other gases. 1859 Armand Fizeau (France) determines that the speed of light in water is affected by the flow of water. 1859 Julius Plücker and Johann Hittorf (Germany) discover that cathode rays bend under the influence of a magnet. 1861 James Clerk Maxwell produces the first color photograph by photographing a subject through red, yellow, and blue filters, then recombining the images. 1861 Robert Wilhelm Bunsen and Gustav Kirchoff (Germany) conclude from their experiments that the Fraunhofer lines in the solar spectrum are due to the absorption of light by the atoms of various elements in the sun's atmosphere. 1865 James Clerk Maxwell determines mathematically that electromagnetic waves travel at the speed of light. He doesn't believe this to be a coincidence and concludes that light is a form of electromagnetic wave. This confirms Michael Faraday's insight (1846) but still requires experimental proof.

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