Voigt & Hothgesang, G&ttingen. 139 



II. 



Prof. O. Lehmann's Crystallization Microscope 1 ) (Fig. 2) for 

 observing the formation of crystals at high temperatures, also during 

 incandescence and electrolysis. 



The stand is fitted with two stages revolving about the optical 

 axis. The lower stage is completely covered and is divided in 360. 

 The upper stage may be adjusted independently of the lower stage. 



Three mirrors serve as polarizer, two of these being fixed, while 

 the illuminating mirror, for quickly changing the illumination, is capable 

 of rotation. 



The burner is firmly attached to the foot but can be moved aside; 

 the air and gas supply is regulated by two screw-valves. Several blow- 

 tubes may be attached to the stage for rapidly cooling preparations or 

 objectives. For the examination of incandescent preparations a special 

 objective is used; it is provided with a double casing through which a 

 constant stream of cold water is made to flow. The preparation is placed 

 upon a separate stage and rests upon 4 thin platinum points, whereby the 

 loss of heat by conduction is minimized. The specimen may also, by means 

 of two platinum wires, be connected with two ebonite mercury cups which 

 are connected with the poles of a small battery. For electrolytical ex- 

 periments at low temperatures another stage, supplied with the microscope, 

 is used. If required the instrument may also be provided with a polariz- 

 er fitted with a lens combination. In this form it may also be used as 

 a mineralogical microscope proper. 



~ III. 



A Collection of 115 thin sections of minerals of petro- 

 graphic interest, prepared with special regard to the system of crystals 

 and mounted according to crystallographic directions. The collection 

 contains also crystals with certain surfaces for goniometric measurements. 



This collection was compiled according to a pattern [collection 

 arranged by Prof. Klein. 



IV. A Collection of 100 thin sections of eruptive-rock types. 

 To each section is appended a sample of the corresponding rock provided 

 with one polished surface. 



This collection is compiled according to Eosenbusch, Microscop. 

 Physiogr. 1887 and may be supplemented by 250 other rock-specimens. 



1 ) vid. 0. Lehmann. Die Krystall analyse 1890; 0. Lehmann. Molecular- 

 physik L 1888. II. 1889. 



