Textfield Validation With For Loop

well i have one form, which having somewhere around 60 checkboxes and 28 textfields + 3 text areas, submit button

textfields start with name="textfield1" till "textfield28"

like

<input name="textfield1" type="text">
.........
<input name="textfield28" type="text">

now i have made one for loop to check value shouble be numeric.

my for loop

for (i=0; i<28;i++)
{
if(checknumber(document.form1.textfield[i].value)==false)
{
alert ("Please Insert Numerics only");
return false;
}}

checknumber is another function which checks for numerics.. my problem is, i m not able to increment the textfield1..28 with for loop. this for loop should work for the bunch of that 28 textfields only.. it should not take any other element in form.