How to get a list of the desktops in applescript

I'm trying to make an applescript that let's me change the desktop picture to a random picture in a folder on my hard drive

tell application "Finder"
    set desktopPictures to folder "Path:To:Desktop:Pictures:"
set fileList to name of every file of desktopPictures
set theNum to random number from 1 to (count fileList) with seed ((time of (current date)) * 4)
set fileName to item theNum of fileList
set desktop picture to file fileName in desktopPictures
end tell

So far it works perfectly, the only issue I have is when I connect another monitor, his desktop picture won't change. I tried solving this problem with the following code I found making a web search

tell application "Finder"
    set desktopPictures to folder "Path:To:Desktop:Pictures:"
    set fileList to name of every file of desktopPictures
    set theDesktops to a reference to every desktop 
    repeat with aDesktop in theDesktops
        set theNum to random number from 1 to (count fileList) with seed ((time of (current date)) * 4)
        set fileName to item theNum of fileList
        set picture of aDesktop to file fileName in desktopPictures
    end repeat
end tell

But this code won't compile as I get a syntax error saying:

Expected class name but found property. With desktop highlighted on row 4

Solution

You have omitted the tell application "System Events" block from the code you found.

In Applescript some commands exist in the dictionary of specific applications and must be referenced with a 'tell application' block. In this case the 'every desktop' call is in the "System Events" app.

Try this.

tell application "Finder"
    set desktopPictures to folder "Path:To:Desktop:Pictures:"
    set fileList to name of every file of desktopPictures
    tell application "System Events"
        set theDesktops to a reference to every desktop
    end tell
    repeat with aDesktop in theDesktops
        set theNum to random number from 1 to (count fileList) with seed ((time of (current date)) * 4)
        set fileName to item theNum of fileList
        set picture of aDesktop to file fileName in desktopPictures
    end repeat
end tell