If $y=1, u=\frac{x}{x^{2}+1}$ and $v=\frac{-1}{x^{2}+1} .$ It follows that $u^{2}+v^{2}=\frac{1}{x^{2}+1}=-v$ and so $u^{2}+\left(v+\frac{1}{2}\right)^{2}=\left(\frac{1}{2}\right)^{2} .$ This is a circle with radius $r=\frac{1}{2}$ and center at $\left(0,-\frac{1}{2}\right)=-\frac{1}{2} i .$ The circle can also be described by $\left|w+\frac{1}{2} i\right|=\frac{1}{2}.$