3039 
former experiments had been made. The minima were less pronoun- 
ced and with the exception of a single case we could not focus 
on them as accurately as in the preceding case. Sometimes the 
focussing was much more.dubious. Besides the positions of extinc- 
tion were not always the same; sometimes the two edges of the 
lattice which were now lying in a plane perpendicular to the light 
beam, nearly coincided with the directions of vibration of the nicols, 
but sometimes they made with these angles of 30° or 40°. My final 
impression is however, that the double refraction in question really 
exists; but I hope that others will have the opportunity to repeat 
the experiments with better crystals than those that were at my 
disposal. 
