False Colour

Let me introduce the strange circumstances of the very first additive colour photograph, which worked even though it should not have.
In 1802 Thomas Young proposed the theory of colour sensation in humans, suggesting that we have three sets of sensors, detecting red, green and blue. All colour sensations are either single primaries or mixtures in some proportion. This theory was further developed around 1850 by Hermann von Helmholtz, was confirmed physiologically, and is now understood in terms of the three types of cone cells in the retina.
In 1855 the then-young Scottish physicist James Clerk Maxwell proposed, and in 1861 demonstrated, the first photograph taken using this principle. Additive colour displays, still used today in TV screens, combine R,G & B light sources to give full-colour images. (Most photographic colour systems in modern times have worked the other way - in subtractive colour, a multi-layered film removes unwanted colour from white light, either in transmission with slides, or reflection with paper prints.)
Maxwell’s reasoning was that if photographs be taken of a coloured subject sequentially through red, green and blue filters, three monochrome images would result which would record the content of the three primary colours in the subject. If these are then processed to positives they can be projected through the original filters; by using three lanterns and carefully registering the images, the image of the subject should be displayed in its original colours.
The trouble with Maxwell’s experiment was that although it appeared to work, it shouldn’t have. He was using the wet collodion process, which was only sensitive to UV and blue light. He should only have been able to record the blue channel. Maxwell found that his blue exposure was a few seconds, red 8 minutes, and for green he got nothing at all unless he made his filter rather weak and gave a very long exposure. Nevertheless, his result did not look too bad, as we can still see; his colour separations are preserved at the Cavendish Laboratory in Cambridge.
The 1961 centenary of the event was celebrated by Ralph Evans of Kodak, who used the occasion to find out what happened. The sensitivity of the emulsion shows that half of the blue light would be recorded, as would a very small fraction of the green, and no red. However, the dye used to make the red in the ribbon reflects mostly in the red as you would expect, but has a significant secondary peak in the blue/UV region. This secondary peak, present in proportion to the amount of red in the scene, exposes the UV-sensitive wet collodion plate where red is visible to the eye. We now have an explanation for Maxwell’s exposure times as well. The blue was recorded normally, taking only twice the usual exposure for his plate; green was very difficult, relying on a tiny fraction of all the green present; and the red sensitivity was low but usable, as (we now know) there was a significant blue/UV content in proportion to the red.
Maxwell’s method seemed to work because of his choice of subject, but could not then be used in the real world. His 1861 demonstration at the Royal Institution was the first time a colour photograph had been projected, but three-colour photography only became feasible once Vogel developed dyes (in 1873-84) to sensitise emulsions to a wider range of wavelengths. Maxwell’s principles eventually formed the basis of the early successful colour processes a generation after his demonstration, including Autochrome and three-colour cameras such as those of Ives and Miethe.