# Let's begin by building a trivial GUI

 package require Tk
 package require Img
 
 wm geom . +0+0
 grid [text .t -width 20 -height 10 -cursor tcross -font {Courier 12}]
 .t insert end {   this widget
   contains several
   lines of text
   to demonstrate
   how to magnify
   a widget's
   content in 
   another
   widget.
 }
 
# We create an image to serve as the magnifier

 set mag [image create photo -width 192 -height 192]
 
# And we display the magnifier in another toplevel

 toplevel .mag
 wm title .mag Magnifier
 grid [label .mag.l -image $mag]
 wm geom .mag +400+0
 
# We establish a binding to make the magnifying glass update when the mouse moves

 bind .t <Motion> {magnify %W %x %y}

# And the 'magnify' procedure does all the work.

 proc magnify { w x y } {
 
     variable mag
 
     set wid [winfo width $w]
     set ht [winfo height $w]
     if { $x < 12 } {
        set x 12
     }
     if { $x > $wid - 13 } {
        set x [expr { $wid - 13 }]
     }
     if { $y < 12 } {
        set y 12
     }
     if { $y > $ht - 13 } {
        set y [expr { $ht - 13 }]
     }
 
     set from [image create photo -format window -data $w]
     $mag copy $from \
        -from [expr { $x - 12 }] [expr { $y - 12 }] \
               [expr { $x + 12 }] [expr { $y + 12 }] \
         -to 0 0 191 191 \
        -zoom 8
     rename $from {}
 }