ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



649 



To adapt the apparatus to the purposes of microscopic preparations, 

 of a megascope, or of horizontally placed objects, a greater length of 

 optical bench is required. The method of attaining this is shown in 

 fig. 150. Horizontal lateral shelves are drawn out from the ends of the 

 upper part, and by their help a second pair of twin rails is set up and 



:v 



Fig. 150. 



combined with the first, so as to give a total bench length of 2 metres. 

 The two sections of bench are rigidly coupled together. It is found 

 that with the proper combination of Nicols and a black mirror 

 polarisation effects are easily attained, and without slanting the camera. 

 For cooling purposes a suitable water-filled large trough with plane- 

 parallel walls is found to answer well ; in lengthy investigations the 

 water should be renewed. 



