docs: function() {
              if (!this.cachedBoundingSphere) {
                  var box = this.boundingBox();
                  var middle = box[0].plus(box[1]).times(0.5);
                  var radius3 = box[1].minus(middle);
                  var radius = radius3.length();
                  this.cachedBoundingSphere = [middle, radius];
              }
              return this.cachedBoundingSphere;
          }