Physik-Department E 1 0 d e r Technischen U n i v e r s i t z t Miinchen 8046 Garching , Wes t Germany I n t h e v i c i n i t y of a c r i t i c a l p o i n t , a number of i m p o r t a n t p r o p e r t i e s can be d e t e r m i n e d by means of o p t i c a l m e t h 0 d s . l A s a n example l e t u s c o n s i d e r a b i n a r y m i x t u r e near i t s c o n s o l u t e c r i t i c a l p o i n t where t h e observed " c r i t i c a l o p a l e s c e n c e " i s due t o t h e i n c r e a s i n g f l u c t u a t i o n s in c o n c e n t r a t i o n .Prom t h e i n t e n s i t y of t h e l i g h t s c a t t e r e d f r o m t h e s e f l u c t ua t i o n s the c o n c e n t r a t i o n s u s c e p t i b i l i t y w i t h i t s c r i t i c a l exponent y c a n b e o b t a i n e d .The wave v e c t o r dependence of t h e s c a t t e r e d l i g h t p r o v i d e s i n s i g h t i n t o the s i z e of t h e c o n c e n t r a t i o n f l u c t ua t i o n s , L.z., t h e c o r r e l a t i o n l e n g t h 5, w i t h its c r i t i c a l exponent V.S p e c t r a l r e s o l u t i o n of t h e s c a t t e r e d l i g h t shows t h e time s c a l e of t h e c o n c e n t r a t i o n f l u c t u a t i o n s , and i n g e n e r a l a " c r i t i c a l slowing down" of t h e s e f l u c t u a t i o n s is observed on a p p r o a c h i n g the c r i t i c a l p o i n t .The o r d e r p a r a m e t e r , which f o r a b i n a r y m i x t u r e i s the c c n c e n t r a t i o n difference between t h e two c o e x i s t i n g p h a s e s , c a n a l s o , among o t h e r methods, be determined o p t i c a l l y by measuring t h e d i f f e r e n c e i n t h e index of r e f r a c t i o n . F i n a l l y , t h e i n t e r f a c i a l tension i n t h e phase s e p a r a t e d m i x t u r e can b e o b t a i n e d using l i g h t s c a t t e r i n g from i n t e r f a c i a l waves.I n p r i n c i p l e , h e l i u m i s n o t an i d e a l c a n d i d a t e f o r l i g h t s c a t t e r i n g e x p e r i m e n t s b e c a u s e its p o l a r i z a b i l i t y i s q u i t e s m a l l and t h e r e f o r e t h e s c a t t e r e d l i g h t i n r e n s i t i e s a r e v e r y weak.Nevertheless, a l l t h e p r o p e r t i e s mentioned above have been measured i n 3~e -~~e m i x t u r e s near the t r i c r i t i c a l p o i n t ( t e m p e r a t u r e Tt-0. 867K, 3~e -c o n c e n t r a t i o n xt-0. 6752) d e m o n s t r a t i n g t h a t even i
