mod_rewrite: add trailing slash?

I am using a .htaccess file to direct requests for a directory to a custom php file that provides info about that directory (and i want the url displayed by the browser to not change).

Here is the relevant part of the .htaccess file.

RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^(.*)$ /myphp.php?url=%{REQUEST_URI} [QSA,L] 

This works well if you go to a directory and include a trailing slash:

http://domain.com/path/

But without the trailing slash, it doesn't

http://domain.com/path

The url (shown in the browser) turns into:

http://localhost:8888/path/?url=/path

I've tried fixing this by adding a rule above this rule:

RewriteCond %{REQUEST_FILENAME} -D
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ $1/ [L]

But that isn't working for me.

How can I get .htaccess to add the trailing slash if it is omitted and then act just as if it had been there?

Thank you

Update: As requested, here is the whole thing.

<IfModule mod_rewrite.c>
RewriteEngine On

#force trailing slashes on real directories
RewriteCond %{REQUEST_FILENAME} -D
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ $1/ [L]

#use the directory viewer on directories without an index page
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^(.*)$ /myphp.php?url=%{REQUEST_URI} [QSA,L]  

</IfModule>