Document Classes ⇒ doublespacing in Beamer

[vita001]

I tried several ways for doublespacing in Beamer but none of them are working.
In the following code, I use \linespread{2} in the first frame, instead of doubling the line space , it increases the space between math equation and the text. In the third frame, I use \begin{doublespace}\end{doublespace} from the package of setspace. It doesn't even compile.
Thanks.

Code: Select all

 
\documentclass[compress]{beamer}
\mode<presentation> {
 \usetheme{Warsaw}
  \usefonttheme[onlymath]{serif}
  \setbeamercovered{transparent}
  \setbeamertemplate{headline}[default]
  \setbeamertemplate{footline}[default]
}

\usepackage[english]{babel}
\usepackage[latin1]{inputenc}
\usepackage{times}
\usepackage[T1]{fontenc}
\usepackage{mathptmx}
\usepackage{helvet}
\usepackage{courier}
\usepackage[font=small,labelfont=bf]{caption}
\usepackage{subfigure}
\usepackage{fancybox}
\usepackage{setspace}
\setbeamertemplate{navigation symbols}{}
\begin{document}
\begin{frame}
\frametitle{A {\it Preconditioned} Smoothing Scheme}
%\doublespacing
\linespread{2}
\textcolor{blue}{\it smoothing steps:} for $j=1,2,\ldots,m$, 
    $$\hat{z}_j\leftarrow \hat{z}_{j-1}+\lambda_k(\psi_k-A_k\hat{z}_{j-1}),$$
    need doublespacing for this line; need doublespacing for this line; need doublespacing for this line
    \vspace{\baselineskip}
\end{frame}
\begin{frame}
\frametitle{A {\it Preconditioned} Smoothing Scheme}
\textcolor{blue}{\it smoothing steps:} for $j=1,2,\ldots,m$, 
    $$\hat{z}_j\leftarrow \hat{z}_{j-1}+\lambda_k(\psi_k-A_k\hat{z}_{j-1}),$$
    need doublespacing for this line; need doublespacing for this line; need doublespacing for this line; 
    \vspace{\baselineskip}
\end{frame}

\begin{frame}
\frametitle{A {\it Preconditioned} Smoothing Scheme}

\textcolor{blue}{\it smoothing steps:} for $j=1,2,\ldots,m$, 
    $$\hat{z}_j\leftarrow \hat{z}_{j-1}+\lambda_k(\psi_k-A_k\hat{z}_{j-1}),$$
    \begin{doublespace}
    need doublespacing for this line; need doublespacing for this line; need doublespacing for this line; 
    \end{doublespace}
    \vspace{\baselineskip}
\end{frame}
\end{document}

[vita001]

I solved the problem but I don't know why.

Code: Select all

\begin{frame}
\frametitle{title}
\setstretch{1.667}
We have
$$A+B+C=D$$
bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
bla bla bla bla bla bla bla bla bla blabla bla bla bla bla bla bla bla bla bla\\   
\end{frame}

notice the double slash at the end. Without it, \setstretch{} doesn't work.
If someone knows why, please explain to me.