slowing the fading transition to a speed slower then slow

[Brian]

I would like to slow the transition time down...slow is too fast. I hope to
achieve a fading in and out of the slide at a very slow spead....can you
help?

 

[tohlz]

Do you mean fade through black or fade smoothly?
If its fade through black,
You might want to try making use of custom animation.
Draw a big rectangle so that it covers the whole slide and set the fill to
black.
Right click on the rectangle and go to Custom Animation.
Add Effect > Entrance > Fade. Set the fade to very slow.
Now, Go to Slide Show > Slide Transition.
Set to no transition since you have replaced it with the custom animation
already.