Whitespace Trouble In Form Element Names W/javascript

I am having difficulty using javascript to validate form fields with whitespaces in the element names (ex: First Name, Last Name).

here's a code snippet:
###
function validate(formObj) {
if (document.form1.Payment Method.checked){
do something here...}
###

the problem is that javascript won't read the element "Payment Method" unless i mash the name into one word. (i don't want to do that because i later use the element names to provide a printable customer receipt

i tried using the ascii octal number for a whitespace in the above code (...form1.Payment40Method.checked...), but to no avail.