var imgHeight = 294; // set the height of the images
var scrollLayer = "beforeAndAfter"; // set the name of the scrolling layer

function previous(){
  document.getElementById(scrollLayer).scrollTop -= imgHeight;
  document.getElementById('nextLink').style.visibility = "visible";
  
  if (document.getElementById(scrollLayer).scrollTop == 0){
    document.getElementById('previousLink').style.visibility = "hidden";
  }else{
    document.getElementById('previousLink').style.visibility = "visible";
  }
}

function next(imgNum){
  document.getElementById(scrollLayer).scrollTop += imgHeight;
  document.getElementById('previousLink').style.visibility = "visible";
  
  if (document.getElementById(scrollLayer).scrollTop == (imgHeight * (imgNum - 1))){
    document.getElementById('nextLink').style.visibility = "hidden";
  }else{
    document.getElementById('nextLink').style.visibility = "visible";
  }
}