IV. DRAWINGS. 41 



167. The principal Problems of Descriptive Geo- 

 metry, represented by stereoscopic figures, by Julius Schlotke. 



L. Friederichscn and Co., Hamburg. 



168. Stereoscopic representation of a number of the most 

 important crystals, their combinations, &c., by Julius Schlotke. 



L. Friederichsen and Co., Hamburg. 



f? The stereoscopic figures of Schlotke are as yet the only ones of their kind 

 in use for illustrating instruction in descriptive geometry and crystallography, 

 in polytechnic and other higher educational institutions, where they are much 

 appreciated. 



More particularly the division of crystallography is recommended, as it 

 renders unnecessary the usual expensive models, and, better than those 

 models, demonstrates the combinations and growth of crystals. 



A stereoscopic apparatus is placed near the objects. 



