DirectX C# .net Quaternion ArcBall Cumulative rotation

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C#

Been playing around with a 3D example and having trouble getting the rotation to be cumulative. The example is just a 3D box. I am trying to add ArcBall rotation to it via a quaternion and mouse click points. It works well with the exception of the Box resetting on the next click. I am no DirectX expert and do not fully understand matrix manipulation. I think what I need to do is set the rotated matrix as the current world matrix so that the succeeding matrix manipulation will become cumulative. Although I may be way off? But if so how do I do this? The Microsoft on line docs are pretty vague and it is kind of driving me nuts. Hopefully someone can help me out.

This is the Mouse Down event

if (e.Button == MouseButtons.Left)
{

    HalfWidth = this.ClientRectangle.Width / 2.0f;
    HalfHeight = this.ClientRectangle.Height / 2.0f;

    MouseDownX = (e.X - HalfWidth) / HalfWidth;
    MouseDownY = (HalfHeight - e.Y) / HalfHeight;

    mouseroll = true;
}

This is the Mouse Move event

float curX = (e.X - HalfWidth) / HalfWidth;
float curY = (HalfHeight - e.Y) / HalfHeight;


// Rotate on Right Mouse Button Drag
LastWorldRotation = WorldRotation;
WorldRotation = ComputeRotation(MouseDownX, MouseDownY, curX, curY); // We Compute the New World Rotation

This is the Compute rotation function

public static Quaternion ComputeRotation(float p1x, float p1y, float p2x, float p2y)
     {
         Vector3 axis;  // axis of rotation
         Vector3 p1;
         Vector3 p2;
         Vector3 d;
         float phi;     // how much to rotate about the axis
         float t;

         if ((p1x == p2x) && (p1y == p2y))
         {
             // Zero rotation
             return new Quaternion(0, 0, 0, 1.0f);
         }

         //
         // First, figure out z-coordinates for projection of P1 and P2 to ball
         //
         p1 = new Vector3(p1x, p1y, p_ProjectToSphere(TRACKBALLSIZE, p1x, p1y));
         p2 = new Vector3(p2x, p2y, p_ProjectToSphere(TRACKBALLSIZE, p2x, p2y));

         //
         // Now, we want the cross product of P1 and P2
         //
         axis = Vector3.Cross(p1, p2);

         //
         // Figure out how much to rotate around that axis.
         //
         d = p1 - p2;
         t = d.Length() / (2.0f * TRACKBALLSIZE);

         //
         // Avoid problems with out-of-control values...
         //
         if (t > 1.0f) t = 1.0f;
         if (t < -1.0f) t = -1.0f;
         phi = 2.0f * (float)System.Math.Asin(t);


         return Quaternion.RotationAxis(axis, phi);
     }

     private static float p_ProjectToSphere(float r, float x, float y)
     {
         float d, t, z;

         d = (float)System.Math.Sqrt(x * x + y * y);

         if (d < r * 0.70710678118654752440)
         {
             // Inside sphere
             z = (float)System.Math.Sqrt(r * r - d * d);
         }
         else
         {
             // On hyperbola
             t = (float)(r / 1.41421356237309504880);
             z = t * t / d;
         }

         return -z;
     }

And this is my Render function

private void Render()
       {
           d3dDevice.Clear(ClearFlags.Target | ClearFlags.ZBuffer,
                            Color.White, 1.0f, 0);


           d3dDevice.BeginScene();

           rotation_matrix = Matrix.RotationQuaternion(WorldRotation);

           world_matrix = Matrix.Identity;

           translation_matrix = Matrix.Translation(0.0f, 0.0f, 5.0f);

           world_matrix = rotation_matrix * translation_matrix;


           d3dDevice.SetTransform(TransformType.World, world_matrix);
           d3dDevice.SetTransform(TransformType.View, world_matrix);
           d3dDevice.SetTransform(TransformType.Projection, world_matrix);

    //       d3dDevice.Transform.World = world_matrix
           d3dDevice.Transform.World = Matrix.Identity;

           d3dDevice.Transform.Projection = Matrix.PerspectiveFovLH(Geometry.DegreeToRadian(45.0f), (float)this.ClientSize.Width / this.ClientSize.Height, 0.1f, 100.0f);

           d3dDevice.VertexFormat = Vertex.FVF_Flags;
           d3dDevice.SetStreamSource(0, vertexBuffer, 0);

           d3dDevice.SetTexture(0, texture);

           d3dDevice.DrawPrimitives(PrimitiveType.TriangleStrip, 0, 2);
           d3dDevice.DrawPrimitives(PrimitiveType.TriangleStrip, 4, 2);
           d3dDevice.DrawPrimitives(PrimitiveType.TriangleStrip, 8, 2);
           d3dDevice.DrawPrimitives(PrimitiveType.TriangleStrip, 12, 2);
           d3dDevice.DrawPrimitives(PrimitiveType.TriangleStrip, 16, 2);
           d3dDevice.DrawPrimitives(PrimitiveType.TriangleStrip, 20, 2);

           d3dDevice.EndScene();

           d3dDevice.Present();

       }