In this experiment, your task will be to rate how dissimilar you think various objects 
look.  At the start of each trial, the shutter will open and you will see two cards: one on 
the right and one on the left.  The cards are grayscale, potentially ranging from black 
through various grays to white, and have nothing printed on them.  They may be at 
different slants and seen under different illumination. 
  
Your task is to rate on a scale of 0 to 30 how dissimilar the grays of the two cards appear.  
A rating of "0" would mean the two cards appear to be exactly the same gray.  A rating of 
"30" would mean that the two grays appear extremely dissimilar.  At the start of the 
experiment, we will show you an example of a pair of cards that we would like you to 
rate as 30. 
  
To make your response, you will use the slider in front of you.  The screen in the middle 
will have the number corresponding to the current slider position.  Sliding the bar to the 
right will increase the value, while sliding the bar to the left will decrease the value.  
When the number on the screen matches your rating of the two cards, press the button. 
 
Before you started this experiment, we had you examine wooden blocks and to notice 
that there can be a difference between how much light appears to reflect from objects on 
the one hand and how the objects are painted on the other. 
 
[A] In this experiment, we would like you to judge the reflectance of the cards, rather 
than the amount of light that appears to reflect from them.  That is, imagine that the two 
cards are presented side-by-side at the same slant under a common illuminant, and judge 
how similar they would look in that situation. 
 
[B] In this experiment, we would like you to judge how much light appears to reflect 
from the cards, and not the reflectance of the cards or how you think they would look if 
they were presented side by side at the same slant.  Finally, in making your dissimilarity 
judgments we would like you to ignore any differences in illumination between the two 
chambers of the apparatus. 
 
It is now time to begin the experiment.  If you have any questions at this point or any 
time during the experiment, please ask the experimenter.  Information regarding the 
current trial number will be visible on the upper left corner of the monitor.