The reason is that the source of light (the candle flame) is drawn out in a vertical direction. When the fork is set vertically, the boundary between light and shadow for each of the prongs on the screen is in approximately the same place for all points of illumination from the source, and therefore the shadow of the prongs is distinct. But when the fork is set horizontally, then the boundary between light and shade made by one point in the source of light with a given prong will be displaced on the screen relative to the boundary between light and shade made with the same prong by another point in the light source; and so the whole shadow cast by the fork will be blurred.