大神能帮我看一下为什么点击看不清无法刷新

  <tr><td align="center">验证码</td><td colspan="3"><input type="text" name="authcode"> 

    <img  id="captcha_img" border="1" src="./captcha.php?r=<?php echo rand();?>"width=100px>

    <a href="javascript:void(0)" onclick="document.getElementById('captcha_img').scr='./captcha.php?r='+Math.random()">看不清</a> </td></tr>