Many people don't seem to know that the lenses in the human eye start going yellow with age. By the age of 50 most people are looking at the world through yellow filters! I experienced this when having a lens replaced because of cataract. The new plastic lens was clear, and when I came out of the operating room, I could see that the dresses of the nurses (that were grey/white stripes before) were now blue/white when I looked at them with the new lens. With the yellow lens in my other eye they were still grey/white! A blue sky was now qite vivid with the new plastic lens, not so with the old yellow lens. I played about with Wratten filters to see how yellow my original lens was, but sadly I can't find the details any more, and nnow have two clear plastic lenses. I wonder how old the 'Standard Observer' was? Bob Frost