The default way of displaying a slide after another during a show is simple to switch to it instantly. Instead, you can apply a gradual merging of one slide with the next.
Select Slide Transition from the Slide Show menu.
Click on the drop down in the Effect box. Choose a transition from the list – a preview of the effect will be shown. Select the transition speed to be Slow, Medium or Fast.
Choose On mouse click to manually move from one slide to the next
Choose Automatically after and set a time in minutes and seconds (usually you’ll just want seconds here).
Note that both these options can be set – if you click before the time is up, the slide show will move on.
Click the Apply button to make the change on the current slide only and Apply to all to affect every slide in the show.