"For approximately the first 10 minutes in the dark, the cones require less light to reach a threshold response than do the rods. Thereafter, the rods require less light. The point at which the rods become more sensitive is called the rod-cone break."
http://www.yorku.ca/eye/darkada1.htm

In other words, it takes 10 minutes in a dark room before our eyes start being able to see in the dark. It takes an additional 20 minutes to become fully adapted to the lower levels of light.

For purposes of discussion here, concerning what Fleet and John did or did not see in the WR, it is important to point out that one must be IN a dark room in order to adapt to the darkness. It simply will not happen if one remains in a lighted room looking into a darkened room. Driving for 10 minutes or less, from one house to another at night as FW did when he drove to the Ramseys at 6:00 am on the 26th., will not cause the eyes to shift into dark adaptation, due to the light levels - reflected moonlight, car head lights, street lights, other car headlights, stop lights and lighted signs all contributed to the amount of light available to the eyes of FW. His eyes began to adjust, but only to the level necessary to see what he had to see, not to the level we would expect to find in a room with no internal light sources (assuming of course that the light switch had not been turned on). If the drive took 10 minutes or less, whatever shift toward the rod-cone break point his eyes may have made would have been canceled out within 5 minutes of arriving at the Ramsey house. That is the average time it takes for the eyes to adjust from a dark room to bright sunlight. FW was certainly at the house more than 5 minutes when he found his way to the WR and looked into it from the threshold.

Consequently, the argument that somehow FW's eyes had adapted at that point, from the drive over and disregarding his time inside the house, such that he would have seen anything in the WR IF it had been there, proving that John's ability to see "something white on the floor" of the WR was evidence of pre-knowlege due to guilt rather than what he would have routinely seen and reacted to. But rather than exposing John's guilt, this argument exposes a misunderstanding and misapplication of eyeball physics.

The next time you drive 20 minutes at night to a neighborhood cinema and stand in line for another 10 minutes in a lighted lobby to buy tickets and snacks, see how adapted your eyes have become when you finally walk into the theatre. The only way you will see anything for the first 10 minutes or so will be from the reflected light of the movie projection onto and from the screen. This would be very close to the experience of FW's as he opened the door to the WR, except the reflected light from the hallway he was in and the furnace room was not very bright and his large frame blocked most of that as he stood in the doorway. Even if he had entered the room he would have had to stand there a few moments until his eyes adjusted to the light entering the room. Conversely, as stated before in my earlier post on this subject, there was more ambient light from the furnace room (daylight) and more shinning past John (he was smaller and shorter) as he entered the WR. He entered and stepped to the right to reach the switch allowing for more light inside the room. Dark adaptation did not occur in either man, either time.

The eyes "see" in the dark by comparing contrasts, degrees of whiteness and blackness. Adapting to the dark means that our eyes become more sensitive to contrasts, able to differentiate between "shades of gray". Put another way, the easiest thing for anybody to recognize in the dark would be something white. What John Ramsey saw, and what Fleet White did not see, is perfectly understandable in context.

See also:

"Heightened sensitivity to light when the eye is subjected to darkness for an extended period. Chemical changes take place in the retina, mostly in the first 20 minutes in darkness but continuing for up to two hours, that greatly improve the observer’s ability to see faint objects."
http://www.daviddarling.info/encyclopedia/D/dark_adaptation.html