$n: 16;
$d: .375em;

.ring {
  position: absolute;
  top: 50%; left: 50%;
  margin: -$d/2;
  width: $d; height: $d;
  border-radius: 50%;
  animation: rot .5s linear infinite;
  
  @for $i from 0 to $n {
    $m: ($i + 1)*4;
    $ba: 360deg/$m;
    $r: ($i + .5)*3*$d;
    
    $sh: ();
    
    @for $j from 0 to $m {
      $sh: $sh, $r*cos($j*$ba) $r*sin($j*$ba);
    }
    
    &:nth-child(#{$i + 1}) {
      box-shadow: $sh;
      animation-duration: pow($i + 1, 1.5)*.5s;
    }
  }
}

@keyframes rot { to { transform: rotate(360deg); } }