'display [image]' fails on Mac OS X 10.5.2

Command line 'display logo.gif' (for example) results in the 'display - 
help' being printed out as though there was a 'usage' error. Is it  
perhaps that  ImageMagick cannot find the X11 server?

X11 is working as expected: 'xpdf' works; Gimp works, so do  
GhostScript and GhostView. So there would seem to be nothing wrong  
with the installation of X11. It is located in /usr/X11 and there is a  
symlink /usr/X11R6 pointing to /usr/X11

ImageMagick works in so far as the command 'convert  logo: logo.gif'  
produces the logo.gif file in the home directory, and some Perl  
scripts using the Image::Magick module also function perfectly. So I  
have no reason to suppose that there is anything fundamentally wrong  
with the installation.

The version of ImageMagick is 6.3.3 installed on an Intel Mac running  
OS X 10.5.2.

I would be grateful if anyone can suggest what might be adrift and  
what to do to get 'display' to work.

Alan Fry