The space groups of three isomers of acorone

t h e a p p r o × i m a t i o n . I t is th is t e r m A = C~--4AB which de t e rmines b o t h t he qua l i t y of t he shape of t he p e a k a n d t h e a c c u r a c y w i t h w h i c h the m a x i m u m posi t ion can be d e t e r m i n e d . F o r A < 0 t he a p p r o x i m a t i o n is good a n d will y ie ld a c c u r a t e resul ts p r o v i d e d A < 0, B < 0. If A < 0, b u t A > 0 a n d B > 0 t h e n the wrong po in t has been chosen as Z(0 , 0) a n d the p e a k o b t a i n e d wil l be sh i f ted f rom the t r ue pos i t ion b y an a m o u n t p ropor t iona l to t he dis tort ion in t he e l l ip t ical pa rabo lo id . This has been shown g raph i ca l l y b y Dickens . As is co r rec t ly exp la ined b y D ickens , t h e m a x i m u m posi t ions found us ing each of the p o i n t s 616, 627, 634 in t u r n as Z(0, 0) shou ld no t be e x p e c t e d to coincide. H o w e v e r , D ickens ' use of these m a x i m u m posi t ions as a measu re of the a c c u r a c y of t he :L.K. a p p r o x i m a t i o n is i nva l id since t he a p p r o x i m a t i o n o n l y holds w h e n the h ighes t po in t , i.e. 645 is used as Z(0 , 0). D ickens ' m e t h o d of check ing the a c c u r a c y of the L . K . m e t h o d b y choosing poin ts o the r t h a n the h ighes t po in t on t he m a p as Z(0, 0) of ten will lead to subs t an t i a l l y incor rec t values . This can be shown by e x a m i n i n g the