Can't get referencing element by id to work in mozilla

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I'm having trouble getting this seemingly simple bit of dynamic HTML to work
under Mozilla 1.4. It works fine under IE6 and in Opera 7, so I can't be too
far off. Been playing around with this for hours now and searching the web
for answers, and I'm obviously missing something here, as Mozilla is
generally very standards compliant.

The code is suppost to swap the background image in the top cell for a
different image for the onMouseOver event. The code will execute if I change
the reference from r4.className="" to this.className="" (but obviously
change the wrong item). So how do I get it to change the background of
another element as I'll be wanting to extend this onMouseOver to affect
several elements in a larger image map.

Is there another method to achieve the same result?

Many thanks in advance.


.earth-r4-1 { background-image: url(images/title/earth_r4_c1.jpg); }
.earth-r5-1 { background-image: url(images/title/earth_r5_c1.jpg); }

<td width="270" class="earth-r4-1" id="r4">&nbsp;</td>
<td valign="middle"><a href="introduction.htm" class="titlepagelink"
   onMouseOut ="r4.className='earth-r4-1'">


Rules are written for those who lack the ability to truly reason,
But for those who can, rules become nothing more than guidelines,
And live their lives governed not by rules but by reason.
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