[Unity C#] Raycast & LayerMask

float myVisionDistance = 50;
Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
RaycastHit hit;
int t_layerMask = (int) Mathf.Pow (2, 8); //for the layer you want to do the raycast
if (Physics.Raycast (ray, out hit, myVisionDistance, t_layerMask))
if (hit.collider.tag == "Food") {
    myTargetPosition = hit.transform.position;
}

Calculate layerMask:
If you want to do the raycast on layer 1,4&8,

    layerMask = (int) (
        Mathf.Pow (2, 1) + 
        Mathf.Pow (2, 4) + 
        Mathf.Pow (2, 8)
    )