Image will not switch between hidden and show

I cannot get an image to switch back and forth between 'hidden' and 'show'

I'm using ideas from How to create a hidden <img> in JavaScript?

I have two different buttons, trying one using html and the other to use javascript - If I comment out one line, the light bulb photo is displayed

 //document.getElementById("light").style.visibility = "hidden";

That line of code is in my 'init' function

If I do not comment the line, the light stays 'hidden', no matter which of the buttons I click

I don't see any errors in the Web Console Log in Safari

 <!DOCTYPE html>
 <html>
 <body>

 <h1>Switch on the Light</h1>

 <img id="light" src="WebVuCoolOldBulb-2.jpg" style="width:100px" >

 <button type = button

 onclick="document.getElementById('light').src.show ='WebVuCoolOldBulb-2.jpg'"  >Switch On the Light

 </button>
   <input type="button" id="onButton" value="ON" />

 </body>

 <script>

 //document.images['light'].style.visibility = hidden;

  function init() {
     //document.getElementById("light").style.visibility = "hidden";
     var onButton = document.getElementById("onButton");
     onButton.onclick = function() {
        demoVisibility() ;

     }
   }

 function demoVisibility() {
     document.getElementById("light").style.visibility = "show";

 }
  document.addEventListener('readystatechange', function() {
   // Seems like a GOOD PRACTICE - keeps me from getting type error I was getting

     // https://stackoverflow.com/questions/14207922/javascript-error-null-is-not-an-object

     if (document.readyState === "complete") {
       init();
     }
   });



 </script>
 </html>

The visibility style property has values of visible and hidden.

There is no show value.

function init() {
     document.getElementById("light").style.visibility = "hidden";
     var onButton = document.getElementById("onButton");
     onButton.onclick = function() {
        demoVisibility();
     }
   }

 function demoVisibility() {
     document.getElementById("light").style.visibility = "visible";
 }