58 SELECTION AND USE 



As eyes differ greatly in their distance apart, the eye-pieces 

 in the binocular are made variable in this respect by being 

 pushed out or in, either by a lever or a rack and pinion. As 

 the bodies are not parallel, but form an angle with each other, 

 it is evident that the further the eye-pieces are drawn out the 

 wider apart do they become. 



Binocular Eye-Piece. A very valuable and efficient 

 means of converting an ordinary monocular microscope into a 

 binocular, has been devised by Mr. Tolles. The one which we 

 possess gives excellent results with objectives of higher power 

 than those available with the Wenham prism as ordinarily con- 

 structed, and it is greatly to be regretted that its high price 

 ($80) places it beyond the reach of those for whom this work 

 is chiefly intended. It has been copied extensively in Europe, 

 but we believe that it is now definitely settled that the honor of 

 the invention belongs to Mr. Tolles. 



The Inverted Microscope. Some years ago, Professor 

 J. Lawrence Smith devised a microscope specially adapted to 

 chemical investigations. In this instrument the stage is placed 

 over the objective, which is inverted, and the rays from the 

 object, after being reflected down through the objective, enter 

 a prism, which so changes their direction that they pass up the 

 body to the eye-piece, and thence to the eye of the observer. 

 For very many investigations the inverted microscope is a very 

 useful instrument. 



Lithological Microscopes. The application of the 

 microscope to petrology, or the study of rocks, has now become 

 such an important department of scientific research that micro- 

 scopes specially adapted to this purpose are manufactured by 

 several makers. These microscopes possess several very im- 

 portant conveniences, which are almost indispensable for such 

 work, but which the limits of the present volume forbid us to 

 describe at length. 



The Aquarium Microscope. The importance of the 

 compound microscope in the examination of small portions (in 

 situ] of large objects, without any preparation whatever, has led 



