ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



229 



large and a small 



smaller motion. The milled head is made with a 

 diameter, so that for moderate powers the small milled head can be 

 rapidly revolved, thus giving a quick motion ; the larger milling enables 

 full advantage to be taken of the delicate adjustment with high powers. 



Fig. 34. 



Societe G-enevoise : Mineralogical and Petrographical Microscopes, 

 with Permanent Centring and with Objective Rotation. — A section 

 of this instrument, numbered 2426 in the maker's catalogue, is shown in 

 fig. 35. The system has the advantage of remaining always centred. 

 The stage carries a column on which slides the objective-holder, and to 

 this latter the objective is applied by means of a spring clamp, which 

 facilitates rapid change of objective. The Microscope tube is mounted 

 on a strong column and moves independently of the objective. There is 

 an opening in the tube above the objective for inserting optical lamellae 

 or for a revolver of plates of mica and quartz. 



Fig. 36 shows model No. 2429 of the same firm. The purpose of 

 the instrument is the same as with the last, and similar advantages are 



* Catalogue of the Societe Genevoise pour la construction d'instruments de 

 physique et de mecanique, 1907. 



