In [2] (page 27) Engelking states that: 



> 
> The Niemytzki plane was defined (and attributed to Niemytzki), by Alexandroff and Hopf in [1]
> 
> 
> 


which is accurate since in [1], where the plane is defined there is a footnote attributing that example to Niemytzki.


Does anyone know whether there is an article or book by Niemyzki where he defines the plane? 


Bibliography: 


[1] Alexandroff, P., Hopf, H., *Topologie I*, Berlin 1935.


[2] R. Engelking, *General Topology*, Heldermann Verlag, Berlin, Sigma Series
in Pure Mathematics 6, 1989.



Is there an article/book where Niemytzki defined his plane?
V. Niemytzki, [Über die Axiome des metrischen Raumes](http://gdz.sub.uni-goettingen.de/dms/load/img/?PID=GDZPPN002274779&physid=PHYS_0674) (1931) [see page 670]


