The root directory contains both the boot program and the file containing the kernel for the system. The name of this kernel file varies from one manufacturer's machine to another, but will usually include the letters nix so you can search for it with wildcard characters. For example:
ls /*nix* -rwxr----- 1 root 1558734 Sep 24 1992 /vmunix
This lists details of any files in the root directory with the characters nix in their name. In this example it lists vmunix - a common name for the kernel file.