src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public static Observable<Long> interval(long period, TimeUnit unit, Scheduler scheduler) {
        return interval(period, period, unit, scheduler);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public static Observable<Long> interval(long period, TimeUnit unit, Scheduler scheduler) {
        return interval(period, period, unit, scheduler);
    }
Returns an Observable that emits a sequential number every specified interval of time, on a
specified Scheduler.
<p>
<img width="640" height="200" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/interval.s.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>You specify which {@link Scheduler} this operator will use.</dd>
</dl>

@param period
the period size in time units (see below)
@param unit
time units to use for the interval size
@param scheduler
the Scheduler to use for scheduling the items
@return an Observable that emits a sequential number each time interval
@see <a href="http://reactivex.io/documentation/operators/interval.html">ReactiveX operators documentation: Interval</a>