ZOOLOGY AND BOTANY MICROSCOPY, ETC. 



631 



that the spectrum line to he measured coincides with the adjustment 

 mark. The minimum of deviation is thus automatically given ; whilst 



Fig. 156. 



with Fraunhofer's there is required the repeated adjustment and the 

 simultaneous testing of the adjustment of the telescope obtained by 

 the rotation of the prism about its vertical axis. Abbe's method also 



