John Herschel (and hypo) denotes the connection between Sir John F. W. Herschel and his identification (1819) of sodium thiosulfate (“hypo”) as an effective photographic fixer, and the cluster of chemistry, processes, people, publications, and preservation practices that grew from that discovery.
Every lasting image, from the first daguerreotypes to analog film, owes its stability to that 1819 experiment in Herschel’s laboratory.
John Herschel, an English scientist born in 1792, discovered the chemical compound known as “hypo”, sodium thiosulfaten, in 1819.
This discovery changed photography by making it possible to preserve images permanently.
Before Herschel’s work, early photographs faded quickly because the light-sensitive silver compounds continued to react to sunlight.
Herschel found that hypo could dissolve the unused silver salts left on a photographic surface.
This stopped the reaction and fixed the image in place.
His finding gave photographers a reliable way to make their pictures last.
In 1839, when photography was introduced to the public by Louis Daguerre and William Henry Fox Talbot, Herschel shared his earlier discovery with them.
Both adopted hypo as the key step in their methods.
In 1842, John Herschel invented the cyanotype process, a photographic printing method that uses iron salts to produce distinctive Prussian blue images, later popularized for creating architectural blueprints and scientific illustrations.
Herschel’s influence extended far beyond chemistry. He introduced several fundamental terms that still define photographic language today:
"Photography": From the Greek photos (light) and graphein (to draw)
“Negative” and “Positive”: Describing image tones in reversal and reproduction.
“Snapshot”: Used later to describe brief exposures.
Herschel’s 1840 publication, “On the Chemical Action of the Rays of the Solar Spectrum on Preparations of Silver and Other Substances”, explained the photochemical reactions involved in image formation and stabilization.
Herschel’s 1819 experiment bridged chemistry, optics, and photographic science, transforming photography from a fragile curiosity into a practical technology.
Discovery Year: John Herschel discovered the photographic fixing power of sodium thiosulfate (Na₂S₂O₃) in 1819, nearly 20 years before photography was officially introduced to the public.
Common Name-“Hypo”: Herschel referred to the compound as “hyposulphite of soda,” shortened to “hypo.” This name remained standard in photography for over a century.
Purpose of Hypo: Hypo acts as a fixing agent — it dissolves unexposed silver salts from photographic materials, preventing further reaction to light and making the image permanent.
Problem Solved: Before hypo, early photographs faded or darkened when exposed to sunlight. Herschel’s chemical solution stopped this, creating stable, lasting images.
Scientific Principle: Hypo removes light-sensitive silver halides (like silver chloride or silver bromide) that remain after development, leaving behind the metallic silver image.
Shared with Other Pioneers: In 1839, Herschel shared his discovery with William Henry Fox Talbot and Louis Daguerre, who adopted hypo as the standard photographic fixer.
Permanent Photography Begins: Herschel’s discovery made photographic permanence possible, marking the transition from temporary light drawings to preserved visual records.
Terminology Contributions: Herschel also coined the terms “photography,” “negative,” and “positive,” defining the basic language still used in image-making today.
Photochemical Understanding: His experiments explained the chemical action of light on silver compounds, laying the foundation for modern photochemistry and later spectroscopy.
Influence on Photographic Processes: Hypo became an essential component in processes like the daguerreotype, calotype, and later film development, remaining in use well into the 20th century.
No Patent or Profit: Herschel never patented hypo or sought financial gain. He freely shared the discovery, helping photography spread as both a science and an art form.
Legacy: John Herschel’s identification of hypo gave photography its chemical stability. Every surviving 19th-century photograph owes its endurance to this breakthrough.
John Herschel’s 1819 discovery of sodium thiosulfate (“hypo”) provided the first reliable method to fix photographs permanently. By dissolving unexposed silver salts, hypo secured photography’s future, making Herschel one of the key figures in the science of light and image preservation.
| Year | Milestone |
|---|---|
| 1792 | John Frederick William Herschel is born in Slough, England, to astronomer William Herschel, discoverer of Uranus. |
| 1813 | Herschel graduates as Senior Wrangler in mathematics from St. John’s College, Cambridge, marking his early brilliance in analytical science. |
| 1819 | While studying chemical reactions of silver salts, Herschel identifies that sodium thiosulfate dissolves unexposed silver halides, forming a stable image. This discovery lays the foundation for photographic fixing. |
| 1820–1830s | Herschel continues exploring light-sensitive compounds and silver reactions, unknowingly building the groundwork for future photographic chemistry. |
| 1834–1838 | While conducting astronomical surveys in South Africa, Herschel refines his understanding of optics, light, and photochemical behavior. These studies later support his photographic research. |
| 1839 | Louis Daguerre in France and William Henry Fox Talbot in England announce photographic processes. Herschel reintroduces his earlier finding of hypo as a fixing agent to make images permanent. |
| 1839 (January) | Herschel publicly shows that sodium thiosulfate fixes photographic images by removing unreacted silver compounds — the first true “fixer” in photographic history. |
| 1839 (February) | Herschel shares his discovery with both inventors. They adopt hypo in their respective methods — the calotype (Talbot) and daguerreotype (Daguerre). |
| 1840 | In Philosophical Transactions of the Royal Society, Herschel publishes “On the Chemical Action of the Rays of the Solar Spectrum,” explaining how light affects silver and other materials, a key text in early photochemistry. |
| 1842 | Using his chemical expertise, Herschel invents the cyanotype (blueprint) process, another method based on light and chemical reactions, further showing the practical power of hypo chemistry. |
| 1845 | Herschel observes “epipolic dispersion” (later called fluorescence), advancing the understanding of light and photochemical phenomena related to photography. |
| 1850s–1860s | Sodium thiosulfate becomes the universal photographic fixing solution, used in studios and laboratories worldwide for daguerreotypes, calotypes, and later collodion processes. |
| 1864 | Herschel compiles his research into major scientific works, reflecting on optics, light, and the role of chemistry in science and photography. |
| 1871 | Herschel dies in Kent, England, and is buried in Westminster Abbey near Isaac Newton. His discovery of hypo remains one of photography’s most enduring contributions. |
| Late 19th Century | Hypo becomes the universal term for photographic fixer, used by both professional photographers and hobbyists during the rise of chemical photography. |
| 20th Century | Sodium thiosulfate continues as the standard fixing chemical in film processing, ensuring photographic images remain stable after development. |
| 21st Century | Historians credit John Herschel as the scientist who gave photography its chemical permanence through the discovery of hypo — the essential link between light, chemistry, and image preservation. |
John Herschel’s discovery of hypo in 1819 provided the first reliable photographic fixing process, transforming temporary light impressions into permanent images. His chemical insight bridged the gap between scientific experimentation and artistic practice, ensuring that photography could evolve into a lasting record of human experience.
In 1819, two decades before the word photography entered common use, a young English scientist named John Herschel made a discovery that would quietly define the future of image making.
While experimenting with silver compounds in his family laboratory in Slough, Herschel identified a chemical solution that could dissolve unexposed silver salts. The compound,sodium thiosulfate, would later become known simply as “hypo.”
It was a small discovery with extraordinary consequence. For the first time, it offered a way to make light-formed images permanent.
Photographers could expose, develop, and then fix their pictures, preserving them against the bleaching effects of sunlight. The modern photograph, stable, durable, and repeatable, became possible because of Herschel’s experiment.
Herschel’s finding was not immediately recognized for its future importance. The 27-year-old was then better known as a mathematician and astronomer, the son of William Herschel, discoverer of Uranus.
His early chemical work was part of a broad scientific curiosity that ranged from optics to meteorology. But when the photographic process emerged two decades later, his 1819 observation proved essential.
In January 1839, when the French inventor Louis Daguerre announced his daguerreotype process, the challenge of image permanence remained unresolved. Within months, Herschel revisited his old notes, tested his chemical solutions, and demonstrated that sodium thiosulfate could fix silver-based images effectively.
William Henry Fox Talbot, the British pioneer of paper photography, quickly adopted the same method.
Herschel’s calm authority lent scientific credibility to what had been, until then, a craft of experimenters and artists. In correspondence with Talbot, he described the process in technical terms and proposed a simple name for the new art itself: photography, from the Greek for “drawing with light.” He also introduced the words “positive” and “negative,” vocabulary that would become fundamental to photographic language.
The chemistry behind hypo is straightforward. Silver salts, used in early photographic emulsions, are sensitive to light. Exposure transforms them into metallic silver, creating an image. But any remaining unexposed salts continue reacting to light, slowly erasing the picture.
Sodium thiosulfate binds with these residual compounds, dissolving them and allowing the stable silver image to remain. The process was elegant, economical, and, crucially, reproducible.
For a generation of photographers, Herschel’s contribution offered reassurance that their art could outlast the fleeting moment of exposure. Daguerre’s polished plates, Talbot’s calotypes, and later glass negatives all relied on hypo to secure their images.
The chemical’s utility was so widespread that “to fix with hypo” became shorthand for the act of photographic preservation itself.
Beyond chemistry, Herschel’s intellectual curiosity placed him at the center of 19th-century scientific transformation. He saw the new art not as a novelty but as part of a broader investigation into light, energy, and perception.
His writings reveal a scientist as comfortable discussing the refraction of crystals as he was speculating on the aesthetics of image-making.
By the time he published his paper On the Chemical Action of the Rays of the Solar Spectrum in 1840, Herschel had already advanced a framework for photochemistry. He analyzed how different wavelengths affected materials, anticipating later studies in spectroscopy and radiation physics.
His experiments bridged the artistic and the analytical, an approach that defined his career.
Modern photography, from digital imaging to astrophotography, owes part of its lineage to Herschel’s discovery. The principle remains unchanged: light strikes a sensitive surface, a reaction occurs, and a fixing agent preserves the result.
John Herschel never sought fame for hypo. In his later years, he described it as a byproduct of broader investigations into light and chemistry. Yet the permanence of the photographic image, the reason we can still see Daguerre’s Parisian boulevards or Talbot’s latticed windows, rests on that understated insight from 1819.
For Herschel, who spent his life mapping stars and measuring the unseen, it was fitting that he also found a way to preserve the fleeting traces of light on Earth.
Sir John Herschel’s discovery of hypo remains one of the most influential contributions to both science and photography.
What began as a small chemical experiment became the foundation of photographic permanence.
He did not profit from hypo or seek patents; instead, he freely shared his findings with fellow inventors and scientists.
