Here is an answer, using the technique described here, with more detail,
\pgfmathsetmacro{\ex}{0}
\pgfmathsetmacro{\ey}{1}
\draw (\ex,\ey) -- ++(-15:1)
(\ex,\ey) -- ++(15:1);
\draw (\ex,\ey) ++(45:.8) arc (45:-45:.8);
The secret to this is in the last line,
\draw (\ex,\ey) ++(45:.8)
Jumps the draw cursor to "the 45 degree position on a circle of radius 0.8", without drawing anything (achieved by using only ++ and not any -- in the command)
Then, from there, we draw an arc
arc (45:-45:.8);
from 45 degrees to -45 degrees, of what would be a circle of radius .8.
Pretty roundabout way to do this, but still it works fine.
Edit:
My finished eye:
\begin{tikzpicture}
%eye
\pgfmathsetmacro{\eyeSize}{1}
\pgfmathsetmacro{\ex}{0}
\pgfmathsetmacro{\ey}{1}
\pgfmathsetmacro{\eRot}{-10}
\pgfmathsetmacro{\eAp}{-55}
\draw[rotate around={\eRot:(\ex,\ey)}] (\ex,\ey) -- ++(-.5*\eAp:\eyeSize)
(\ex,\ey) -- ++(.5*\eAp:\eyeSize);
\draw (\ex,\ey) ++(\eRot+\eAp:.75*\eyeSize) arc (\eRot+\eAp:\eRot-\eAp:.75*\eyeSize);
% IRIS
\draw[fill=gray] (\ex,\ey) ++(\eRot+\eAp/3:.75*\eyeSize) % start point
arc (\eRot+180-\eAp:\eRot+180+\eAp:.28*\eyeSize);
%PUPIL, a filled arc
\draw[fill=black] (\ex,\ey) ++(\eRot+\eAp/3:.75*\eyeSize) % start point
arc (\eRot+\eAp/3:\eRot-\eAp/3:.75*\eyeSize);
\end{tikzpicture}
I am definitely unfamiliar with both beamer and tikz (do not quite get what the \only are supposed to do) but perhaps this could go in the direction you want:
\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{chains}
\newcounter{count}
% helper macro:
\long\def\GobToSemiColon #1;{}
\newcommand\myPicture{
\begin{tikzpicture}
\begin{scope}[start chain = going below]
\ifnum\value{count}<1 \expandafter\GobToSemiColon\fi
\ifnum\value{count}>3 \expandafter\GobToSemiColon\fi
\node[draw, rectangle, on chain] {display only when counter is between
1 and 3};
\ifnum\value{count}>-1 \expandafter\GobToSemiColon\fi
\node[draw, rectangle, on chain] {display only when counter is
negative};
\ifnum\value{count}<100 \expandafter\GobToSemiColon\fi
\ifnum\value{count}>200 \expandafter\GobToSemiColon\fi
\node[draw, rectangle, on chain] {display only if counter is between
100 and 200};
\ifnum\value{count}<3 \expandafter\GobToSemiColon\fi
\ifnum\value{count}>20 \expandafter\GobToSemiColon\fi
\node[draw, circle, on chain] {only when counter is in the range 3 to 20};
\end{scope}
\end{tikzpicture}
}
\begin{document}
\begin{frame}
\only{\setcounter{count}{-3}\myPicture}
\only{\setcounter{count}{105}\myPicture}
\only{\setcounter{count}{39}\myPicture}
\only{\setcounter{count}{2}\myPicture}
\only{\setcounter{count}{5}\myPicture}
\end{frame}
\end{document}
Best Answer
If a thicker border is desired,