Before the subject enters the room, turn the lamp on and place the three test blocks in the 
left booth. 
 
Give the subject the following instructions: 
 
"In our experiments, we'll be asking you to judge dissimilarities between painted 
grayscale cards.  Before we start the main experiment, we'd like to familiarize you with 
type of judgments we'd like you to make. 
 
Here are three blocks to examine. You can touch them or move them anywhere on the 
table, and notice how their appearance changes with location or orientation. You will 
have two minutes to look at these blocks.  As you look at any of the blocks, you may 
notice that the amount of light reflected from the block appears to change as you move it 
or rotate it with respect to the light source.  On the other hand, the block itself isn't 
changing as you move it - the reflectance of the block itself is staying the same.  I'd like 
you to examine the blocks now and get a sense of how the amount of light reflected to 
your eye appears to change as you move or rotate each block, while realizing at the same 
time that the reflectance of the block itself is constant."
 
Wait for two minutes. 
 
"In the experiments, we will be asking you to judge the dissimilarity between pairs of 
grayscale cards.  When you judge the dissimilarity, please 
 
**** 
 
[A] judge the reflectance of the cards themselves, 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] judge how much light appears to reflect from the cards, and not the reflectance of the 
cards themselves."