Syntax mongodb for skip and limit

Bonjour,

Either my collection of 30 documents.
I would like to read in this collection the 8 documents between document 20 and document 28 of my request.
I wrote:
Var res = Courses.find ({}, {skip: 20, limit: 28}) fetch ()
or
Var res = Courses.find ({}, {skip: 20, limit: 8}) fetch ()

But the result is empty n both cases !
Would anyone have an idea?
Thank you for your reply.

Merci
YC