Apple Motion 2 Tutorials - Manual (Page 170)

170 Tutorial 4 Working With Particles
To change the opacity of the particles:
1 In the Emitter tab of the Inspector, select the leftmost opacity tag on top of the
opacity bar.
The Opacity parameter becomes active.
2 Using the Opacity slider below, set Opacity to 0 percent.
The gradient now appears entirely black, and the particles are no longer visible,
because the gradient is set to 0 percent opacity over the entire bar.
Add two more opacity tags to the gradient.
3 Click twice in the opacity bar to add opacity tags as shown below.
4 Click the leftmost opacity tag to display the spread control between the first two tags.
Note: A spread control is displayed only when a color or opacity tag is selected.