Schulte's Dissertation on Light and its interaction with chemicals
subsequent printing

Johann Heinrich Schulze

c. 1717 - Discovery that powdered silver nitrate darkened upon exposure to light.
This led to experiments, at first in the flask, later on paper,
wherein stencils were placed in the path of the light rays in
order to produce a negative image upon the silver nitrate.

These images were temporary, however, because a way had not been found to stop the action of the light, which always darkened the pictures to black. His findings were
published in 1719 and reprinted in 1727.

A scan of the original book is here at the Martin Luther University

I covered the glass with dark material, exposing a little part for the free entry of light. Thus I often wrote names and whole sentences on paper and carefully cut away the inked parts with a sharp knife. I struck the paper thus perforated on the glass with wax. It was not long before the sun's rays, where they hit the glass through the cut-out parts of the paper, wrote each word or sentence on the chalk precipitate so exactly and distinctly that many who were curious about the experiment but ignorant of its nature took occasion to attribute the thing to some sort of trick." - Johann Heinrich Schulze (trans. © Robert Leggat, 2002)

Schulze at Wikipedia

Silver and Sunlight at Chemical Heritage Foundation

Photography Pioneers at FotoArt | Invention of Photo Chemistry at


c. 1790 - Thomas Wedgwood, a potter, seeks to employ photography to transfer designs to pottery and is the first to attempt to use the camera obscura to create a photograph on paper. These efforts failed, but "contact prints" were made of leafs, etc. by placing the object upon the sensitized paper - what today we call "photograms".

His collaborator, Humphry Davy, wrote and published the findings by Wedgwood as, “An Account of a Method of Copying Paintings upon Glass, and of Making Profiles, by the Agency of Light upon Nitrate of Silver. Invented by T. Wedgwood, Esq.” in 1802.

early silhouette photographs
"Shadow Pictures" or "Sun Pictures" such as this must be like the
originals by Wedgwood and others, who lacked the ability to stop
("fix") the continuing action of light on the silver halide crystals.


First Permanent Photograph

This picture had been lost since 1898 and was rediscovered in 1952;
read about its rediscovery here in The Photographic Journal from that year.
In France, Joseph Niépce, a lithographer, needed a method to transfer drawings to his printing plates. He knew that a type of asphalt (bitumen of Judea) was hardened by the sun. He waxed the paper of the drawings to make them more translucent & placed them face down on the plate, which was coated with the asphaltum. When the light passed through the paper, it hardened the solution, and where the black of the drawing ink blocked the light rays, the asphaltum remained soft & could be removed by a solvent (lavender oil), leaving behind a negative image. He incised, by methods of etching, this negative image in order that it would retain ink for pressure printing. He called these pictures heliographs ("sun" and "picture"). Circa 1824-26, he placed a plate coated with his mixture inside a camera obscura and took the first permanent photograph that did not depend on a contact method. It was of a barn & some rooftops & took all day to develop.
Joseph Niépce at University of Texas - Austin


In 1829, Louis J. Daguerre, a painter investigating photography, approached Niépce in a quest to share information. Initially suspicious, after a few years Niépce eventually approached Daguerre about a business venture, to which he assented. Niepce died four years later, but Daguerre was able to release a perfected photographic system by 1839. This process, called Daguerreotype, involved exposing a silver coated plate to the fumes of iodine, which made it vastly more light sensitive than the asphalt based plates.

Moreover, a chemical had been discovered (sodium thiosulphate) to stop the chemical reaction of the plate to incoming light. The exposed image on the plate was still invisible (a latent image), but it was "developed" by exposure to the fumes of mercury, which causes formation of silver iodide upon the surface of the plate. The areas that were not struck by light were washed away by the "fixer". The creation of an alloy on the surface of the photo gave this type of picture a bas-relief type surface of great detail. However, due to metallic glare, it had to be viewed from the side and copies could not be made.

Obviously, the fumes involved in the process were very dangerous.

Daguerre at The Metropolitan Museum

first daguerrotype photo
This process was an improvement in both detail and speed,
taking about 40 minutes for exposure.



Also in 1839, only three weeks after Daguerre's announcement of his method, Henry Fox Talbot announced in England his "negative-positive" system. Talbot found that he could slow the effect of light upon silver nitrate with a salty solution, and through a friend, had heard about Daguerre's use of sodium thiosulphate, which halted the process.

Talbot sensitized paper with silver nitrate, exposed it to light in a camera obscura, then processed the image, a "negative reversal" of tone. It was this picture that became his negative; placed atop a blank, sensitized sheet of paper and sandwiched between two planes of glass, the negative sheet created an opposite image, a "positive", upon the second sheet. Just as in Niepce's heliotypes, the black "ink", or, in this case, the darkened silver of the original photo, blocks light to the lithographic plate or, in Talbot's method, the second sheet of paper, creating "white".

Black and grey tones, obviously, were produced by the "negative", or original photo, allowing more light to pass through an unexposed, or less exposed, area.


Such prints were made outdoors by means of sunlight:
Mass production began with the negative process

After about a year Talbot had also been able to create a latent image on the negative, meaning there was no need (or ability, without overexposing the picture!) for "checking" the progress of the darkening photograph within the camera or the print between the plates of glass. This quickened exposure time considerably.

Nevertheless, due to the graininess of the paper, even when waxed (like Daguerre did) to increase translucency, the clarity of the pictures was slightly fuzzy, and a more accurate rendition was sought from photography.

Fox Talbot at De Montfort University

Fox Talbot at Encyclopedia Britannica

Fox Talbot Museum

The Wetplate Collodion Process at :
Alternative Photography | Unblinking Eye

Dry Plate Photography at The Light Farm

Richard Leach Maddox:
Microscopy | Astrophotography | Dry Plate



Talbot's Studio
The Fox Talbott Studio


The first positive print made from a
photographic negative

In 1847, Niepce's nephew introduced the use of a glass plate as "backing" for the light-sensitive emulsion, which was ideal for the purpose, being inert, clear and textureless. The silver nitrate was suspended in an egg white mixture. In 1846, collodian, a dangerous mixture containing ether & alcohol, was developed and exploited by the medical profession because it dried into a hard, coating-like substance which could be used to protect exposed wounds.

By 1850, it was used to suspend the silver crystals in place of the egg white mixture, to superior effect. However, photos had to be taken while the plate was still wet (the "wet plate" method) and processed while still damp. This made traveling photography a burdensome, dangerous affair. Nevertheless, exposure times were down to about 5 seconds and the clarity was excellent.

By 1880, collodion was replaced by animal product gelatin, still used in modern films to suspend light sensitive crystals. This allowed the plate to be stored prior to use, thus creating the "dry plate" method, freeing photographers from some of their labor.


Modern Black and White Film

Silver Bromide Crystals Before and After Development
silver bromide crystals

"The Fundamentals of Photography", by C. E. K. Mees

In modern films, "speed", or light sensitivity, is determined by the size of the silver bromide crystals. "Fast" films, therefore, contain large crystals absorbing more light than smaller crystals ("medium speed") or very small crystals ("slow film speed"). The large size of the crystal in the fast film accelerates exposure time, but possesses a "grainy" texture. The small crystals in the slow film record more detail, but lengthen exposure time.

Wikipedia: Silver Bromide | Silver Halide

  In the bromide crystal, light activates an electron of a negatively charged bromide ion, which is raised to a higher energy level in its orbit around the nucleus and "migrates" toward "impurity elements", sometimes called "sensitivity specks". The speck is now negatively charged and attracts a positively charged free silver ion. The chemical processing of the film exaggerates this effect, building upon this accumulation of silver. Areas unexposed by light are washed away in development, leaving a clear area that will print darkly when light is passed through it, using, once again, the negative-positive method first introduced by Talbot.



Maxwell and Color Photography

First Permanent Color Photo, 1861, Maxwell and Sutton

Maxwell and Sutton at Popular Photography

James Clerk Maxwell, who first understood that light was part of the electromagnetic spectrum, took the first color photograph in 1861. He took three separate exposures of his subject, separately using red, blue and green filters to admit only light of that wavelength to the photographic plate. He then took each plate & dyed it the color of the filter that had produced it. Projecting these three identical, but differently colored, images onto a screen and overlapping them, he was able to use the additive system of color mixing to produce many colors.
Modern color film uses the same theory, producing three negatives within a single piece of film by making certain layers sensitive to only a single color and, similarly, using filters to block the presence of certain wavelengths at particular levels. The processing washes away these negatives & replaces them with colored dyes. Perfectly overlapped, these three colored levels combine for a single full color picture.


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