function next_slide()
{
   currentslide += 1;
   document.images['thisslide'].src = slides[currentslide].src;

   if (currentslide > 0)
   {
      document.getElementById('goback').style.visibility = 'visible';
      document.images['previousthumb'].src = thumbs[currentslide - 1].src;
   }
   
   if (currentslide == (slidecount - 1))
      document.getElementById('goforward').style.visibility = 'hidden';
   else
      document.images['nextthumb'].src = thumbs[currentslide + 1].src;
   }

function previous_slide()
{
   currentslide -= 1;
   document.images['thisslide'].src = slides[currentslide].src;

   if (currentslide == 0)
      document.getElementById('goback').style.visibility = 'hidden';
   else
      document.images['previousthumb'].src = thumbs[currentslide - 1].src;

   if (currentslide < (slidecount - 1))
   {
      document.getElementById('goforward').style.visibility = 'visible';
      document.images['nextthumb'].src = thumbs[currentslide + 1].src;
   }
}