if (document.images) 
{
   img1 = new Image();
   img2 = new Image();
img3 = new Image();
img4 = new Image();
img5 = new Image();
img6 = new Image();
img7 = new Image();
img8 = new Image();
   img1.src = "/images/navImages/sprite_tier1-trans.png";
   img2.src = "/images/navImages/sprite_about_tier2.png";
   img3.src="/images/navImages/sprite_programs_tier2.png";
   img4.src="/images/navImages/sprite_research_tier2.png";
    img5.src="/images/navImages/sprite_publications_tier2.png";
   img6.src="/images/navImages/sprite_give_tier2.png";
    img7.src="/images/navImages/sprite_collections_tier2.png";
     img8.src="/images/navImages/sprite_online_exhibits_tier2.png";
     
 }