How Science Works 35 



merit as a method for observing the bits and pieces of 

 complex situations in a meaningful way, let us begin by 

 noticing some potatoes sprouting in a corner of the cellar. 

 Most of them seem to be bending in one direction, and 

 if we look in that direction we notice that the sprouts 

 are pointing toward an open cellar window. We ask our- 

 selves if there is any relationship between the open window 

 and the direction in which the sprouts are growing. What 

 shall we do to find out? First, let's put a piece of plywood 

 over the window so that it becomes as much like the rest 

 of the wall as possible. After a few days the sprouts lose 

 their relationship to the window and seem to be growing 

 straight up. Next, we ask ourselves what it is about the 

 open window that seems to attract the sprouts. Three pos- 

 sibilities suggest themselves. Maybe the fresh air coming 

 through the window is important, maybe it is the heat, 

 and finally it could be the Ught. How shall we separate 

 these effects from one another? We decide to begin by 

 putting a glass sash in the empty window frame to cut off 

 the fresh air but leave the light and a good part of the 

 radiant heat. Again we come back in a few days to find 

 that the sprouts are pointing toward the window just as 

 they were before. Just to make sure that the fresh air 

 might not have the same effect, however, we better think 

 of some way of cutting off the light and arranging for the 

 fresh air to come in. After a bit of fussing around, we 

 arrange some cardboard boxes outside the window in 

 such a way that air can flow through while the light is 

 cut off. Now the potato sprouts grow straight up. We 

 conclude that the fresh air is not important but that either 

 the light or the heat rays (or both) are important. Now 

 we have the problem of separating these two variables 

 from each other. This can be done with suitable filters 

 and if we can lay our hands on enough different filters 



