This could get boring to a lot of people and some of the younger generation probably won't know what I'm taking about. Anyway, about forty five years ago I started in the field of photography. Back then it was the age of film and when someone wanted a print larger than the contact size of the negative, it had to be enlarged. This was done with a photographic enlarger were the negative was placed inside the enlarger in what's called a negative carrier and the distance between the negative carrier and the photographic "enlarging" paper determined the size of the "print". The black and white negative has basically two components, the emulsion side which is actually the image and dull when light is reflected off of it and the backing or base side which is a highly polished film. When this negative was inserted into the enlarger, the heat of the light bulb inside the enlarger would sometimes cause the "film" to buckle and this would cause various areas on the print to be out of focus. To alleviate this problem they invented "glass" negative carriers. With this glass insert, the carrier would sandwich the negative and keep it completely flat, thus getting rid of any kind of focus problems in the print. Beside having four more surfaces to keep clean and dust free so the imperfections would not be transmitted to the print, we now have the problem with the top sheet of glass making perfect contact with the top of the negative or base side of the film. When the pressure is not completely even across the top of the film, "Newton Rings" would appear because of the two highly polished surfaces. The only way to get rid of them was to adjust the pressure inside of the carrier or use something called "anti Newton ring glass". It was sheets of glass that were very finely etched so this effect would be eliminated.
There ya go.
Dennis
