Anybody know what type of drink that is that was shown in 1st scene?

Does anybody know what that drink is that Mrs. Wellington sends to Christopher Tracy in the very beginning of this movie. If the movie were in color rather than black in white, it might be easier for me to determine what it is but I can't tell in black and white. It is some type of drink that has either salt or sugar around the rim of the glass (like a margarita) and I think there is a slice of lime in it but I'm not 100% sure b/c I can't tell what color it is so it could be a slice of lemon or orange for all I know.