--- ray/src/rt/glass.c	1991/11/12 17:09:32	2.1
+++ ray/src/rt/glass.c	1992/10/14 12:27:47	2.4
@@ -46,7 +46,6 @@ m_glass(m, r)		/* color a ray which hit a thin glass s
 OBJREC  *m;
 register RAY  *r;
 {
-	double  sqrt(), pow();
 	COLOR  mcolor;
 	double  pdot;
 	FVECT  pnorm;
@@ -95,8 +94,12 @@ register RAY  *r;
 		if (!(r->crtype & SHADOW) &&
 				DOT(r->pert,r->pert) > FTINY*FTINY) {
 			for (i = 0; i < 3; i++)		/* perturb direction */
-				p.rdir[i] = r->rdir[i] - r->pert[i]/rindex;
-			normalize(p.rdir);
+				p.rdir[i] = r->rdir[i] +
+						2.*(1.-rindex)*r->pert[i];
+			if (normalize(p.rdir) == 0.0) {
+				objerror(m, WARNING, "bad perturbation");
+				VCOPY(p.rdir, r->rdir);
+			}
 		} else {
 			VCOPY(p.rdir, r->rdir);
 			transtest = 2;