Instantiating Prefab from Javascript - BCE0005: Unknown identifier: 'Prefab'

Hello World!

When I try to instantiate a prefab from a script, I receive error message BCE0005: Unknown identifier: 'Sphere1'.

I have made the prefab so I don't understand why I'm getting the error message.

Here's how I made the prefab:

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1. Right clicked inside the "Project View" window->selected create->prefab.

2. Clicked on GameObject->Create Other->Sphere.

3. Dragged sphere in "Hierarchy" window onto prefab in "Project View" window.

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Here's how I'm instantiating the prefab from the script:

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function OnCollisionEnter(collision : Collision) {

var sphere_bullet : GameObject ; sphere_bullet = Instantiate(Sphere1,transform.position, transform.rotation) ; }

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What I've done so far to try and solve the problem:

Checked "Overview: Instantiate" in Unity manual to see if I missed something:

http://unity3d.com/support/documentation/ScriptReference/index.Instantiate.html

Checked Overview: Script compilation (Advanced) in manual after reading some questions similar to mine:

http://unity3d.com/support/documentation/ScriptReference/index.Script_compilation_28Advanced29.html

Found out that Unity compiles scripts in a certain order leaving in "editor" scripts for last.

Tried changing position of prefab and script in case the script was being compiled before Unity knew anything about the prefab.

Created a new project with a new script and prefab made the same way without any other folders or assets.

Searched Unity Answers for people with the same problem but I couldn't find a solution that worked for me.

Other Information:

The script code isn't contained in any other functions.

The only objects in the "Project View" window are the prefab and the script.

I named the prefab "Sphere1".

Any help would be great.