I don't remember reading anything about that. But in my personal experience, the plate reverts to a murky gray, with slightly darkened outlines of the colored areas. The original color of the over-exposed plate is sort of purplish black.
MSf ------- Original Message ------- On Sunday, November 27th, 2022 at 8:54 PM, H LV <hveeder...@gmail.com> wrote: > Wow... > When you say the colours faded do you mean the plate reverted to being > completely black again (i.e. its over exposed state)? > > Harry > > On Sun., Nov. 27, 2022, 2:58 p.m. MSF, <foster...@protonmail.com> wrote: > >> This effect was studied extensively thoughout the 19th and early 20th >> centuries, but in another field. Early researchers in photography noted the >> same effect and more in their experiments with Daguerrotype plates. A >> purposely over-exposed plate would turn very dark. If the plate was covered >> with pieces of colored glass and re-exposed to bright sunlight, the plate >> would reproduce the colors through which the light was filtered. This no >> doubt tantalized photographers with the idea of color photography, but the >> effect would eventually fade and the exposure times and light intensity >> requirements made that impractical. Further, there was no way to fix the >> image. The same effect can be demonstrated with so-called printing-out >> papers, silver chloride emulsions meant to be contact printed from negatives >> without the need for chemical development. >> >> So, basically, this is a demonstration of one field of endeavor not paying >> attention to developments (pun intended) in another. Besides, it's not nice >> to second guess Goethe. >> >> MSF >> >> ------- Original Message ------- >> On Saturday, November 26th, 2022 at 6:41 PM, H LV <hveeder...@gmail.com> >> wrote: >> >>> This is a google english translation of a german article that was published >>> in December in 2021. >>> >>> The Ultraviolet Enlightenment >>> https://docs.google.com/document/d/178aIZp1ts5J1HCvWuZCkdoDvwbzp8tm_xiPGdonvPM8/edit?usp=sharing >>> >>> (The original article is here >>> https://www.faz.net/aktuell/wissen/physik-mehr/entdeckung-des-uv-die-ultraviolette-erleuchtung-17687221.html) >>> >>> It is about Goethe and Ritter in the early 1800s when Goethe encouraged >>> Ritter to look for invisible radiation beyond the violet >>> end of the spectrum given that Herschel had just discovered evidence of >>> radiation below the red end of the spectrum using a thermometer. >>> Ritter was eventually credited with the discovery of UV light using a light >>> sensitive paper. >>> >>> However,as the article explains he did another experiment which was >>> inspired by Goethe's concept of polarity but to this >>> day the results have been dismissed as an error. Goethe predicted that if >>> UV light darkened the photo chemical paper, then infrared light should >>> lighten the same paper. Ritter reported finding this to be the case but >>> because the chemical process is irreversible subsequent scientists >>> have insisted that the _observation_ of lightning must have been an error. >>> Until 2021 no one had even attempted to replicate this simple experiment, >>> but now there is evidence that Ritter was probably correct in his >>> observation. However, a mystery remains as to why the paper should be >>> lightened. >>> (one chemist speculates the silver atoms are being rearranged so their >>> reflectivity changes). >>> >>> I am posting this as another example of how some observations can be >>> prematurely rejected on the basis of opinion instead of a proper follow up >>> investigation. In this case the observation is more than 200 years old! >>> >>> Harry
