THE AID OF THE ACHROMATIC FRINGES. 103 



some adjustments (spectrum coincidences) seemed to separate into two verti- 

 cal and parallel strings of beads. It is natural to refer these to the two indices 

 of refraction of quartz. 



The distinctive and interesting feature of quartz is the appear- /i 



ance of a new set of interferences superimposed on the normal set 

 just referred to. An illustration of this is given in figure 102, (f) 

 where a represents the duplicated normal group and b the new 

 interferences. If the conditions were not elliptic it would be nat- 

 ural to suppose that the new set is due to the interference of the 

 two normal groups side by side, so that the b group is a linear 

 phenomenon of the second order under high dispersion. The pat- |Q7 



tern (fig. 102) is merely one case; very frequently the b group of 

 fringes consists of oblique lines, or lie on one side or the other of a. They 

 are also much modified as to size by enlarging the slit. The b group may even 

 appear alone, closely resembling Fresnellian fringes. 



By using a nicol at the collimator, either the right or left side of the group 

 a could be eliminated, and the b group seemed to appear for an oblique posi- 

 tion of the nicol when the two sides of the a group were distinct. 



In other adjustments, however, two groups of fringes, side by side but in 

 different focal planes and of an entirely different pattern, were obtained, 

 among which the occurrence of three groups was rare. Though I made many 

 experiments I did not come to a definite conclusion as to the phenomenon 

 as a whole. Two groups, of course, are to be anticipated. 



