Honorably discharged members of the United States Armed Forces who are given employment preference by the United States Government, now and hereafter, shall be given preference for appointment and employment in every public department and upon all public works in this State insofar as such preference may be practicable; age, loss of limb or other physical impairment which does not in fact incapacitate shall not be deemed to disqualify them, provided they possess the capacity of skill and knowledge necessary to discharge the duties of the position involved. Provided, that any public department operating on a merit system shall give preferences similar to those given by the United States Government to eligible members discharged from the Armed Forces insofar as such preferences may be practicable.