Accessing class parameters from companion object in Scala

Question

While declaring a class in Scala, we can pass class parameters which are then used by compiler to generate a primary constructor:

class Rational(n:Int,d:Int){
  override def toString = n + "/"+ d
}
//companion object
object Rational{
   def main(args: Array[String]): Unit = {
   val r = new Rational(1,2)
   println(r.toString)
   println(r.n)
  }
}

In toString method, I can access n and d as class members. But when I try to access n from companion object, I get error as:

'Cannot resolve symbol n'

Edit: From link provided for possible answer I can understand the reason. But why is that I can access 'n' and 'd' in toString method?