56 



CONSTRUCTION OF THE MICROSCOPE. 



Magnifying Power of Messrs. Powell and LealancCs 

 Achromatic Object-Glasses. 



Schmidt's goniometer positive eye-piece, for measuring 

 the angles of crystals, is so arranged as to be easily rotated 

 within a large and accurately graduated circle. In the 

 focus of the eye-piece a single cobweb is drawn across, and 

 to the upper part is attached a vernier. The crystals 

 being placed in the field of the microscope, and care being 

 taken that they lie perfectly flat, the vernier is brought to 

 zero, and then the whole apparatus turned until the line 

 is parallel with one face of the crystal ; the frame-work 

 bearing the cobweb, with the vernier, is now rotated until 

 the cobweb becomes parallel with the next face of the 

 crystal, and the number of degrees which it has traversed 

 may then be accurately read off. 



To the most complete instruments a set of eye-pieces, 

 consisting of not less than three, is usually made. These 

 differ in power ; the longest is always the lowest power, 

 and is marked A. Its angular aperture, which determines 

 the size of the field of view, is generally less than that of 

 the others (if constructed on the Huyghenean plan), being 

 limited by the diameter of the bod}'. It is usually about 

 20 degrees. The next eye- piece, or middle power, marked 

 s, and the deepest, c, have more than 30 degrees of angular 

 aperture. 



For viewing thin sections of recent or fossil woods, 

 coal, the fructification of ferns and mosses ; fossil-shells, 

 seeds, small insects, or parts of larger ones ; molluscs or 



