button1_a= new Image();
button1_a.src = "link01_start.gif"
button1_b= new Image();
button1_b.src = "link01_over.gif"
button2_a= new Image();
button2_a.src = "link02_start.gif"
button2_b= new Image();
button2_b.src = "link02_over.gif"
button3_a= new Image();
button3_a.src = "link03_start.gif"
button3_b= new Image();
button3_b.src = "link03_over.gif"
button4_a= new Image();
button4_a.src = "link04_start.gif"
button4_b= new Image();
button4_b.src = "link04_over.gif"