Usage of classes from other project in one solution

I have created a Silverlight Web Application Project.
Visual Studio creates the Silverlight application at the top and a test project (such as SLApplication_Web). If i create a class e. g. for my web service in the test project, how can I use this class in the Silverlight Application? It's not possible to add a reference to the test project. Is the solution to write a webservice, that gives me a new object? I think this is not good.

 Example:
Solution1
- SLApplication (Usage of the class Test.cs here)

- SLApplication_Web
   - Test.cs (Class)
 
Thanks for your help. 
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