How does __proto__ differ from constructor.prototype?

function Gadget(name, color)
{
   this.name = name;
   this.color = color;
}

Gadget.prototype.rating = 3

var newtoy = new Gadget("webcam", "black")

newtoy.constructor.prototype.constructor.prototype.constructor.prototype 

It always returns the object with rating = 3.

But if I do the following:

newtoy.__proto__.__proto__.__proto__

The chain ends up returning null.

Also in Internet Explorer how would I check the null if there is not a __proto__ property?