EYE-PIECES. 43 



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. Across the focus of the eye-piece a single 

 cobweb is drawn, 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 j the frame-work bearing the cobweb, with the ver- 

 nier, 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 oif. 



The Eye-piece. — To the most complete instruments a set of eye- 

 pieces, consisting of three, is generally 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 Hu3'ghean plan), 

 being limited by the diameter of the body. It is usually about 20 de- 

 grees. The next eye-piece, or middle power, marked b, 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 large ones ; molluscs, or the circulation in the frog, &c., the 

 eye-piece a is best adapted. 



For examining the details of any of the above objects, it will be 

 advisable to substitute the eye-piece b, which also should be used in 

 the observation of crystals when illuminated by polarised light, the 

 pollen of flowers, minute dissection of insects, the vascular' and cellu- 

 lar tissues of plants, the Haversian canals and lacunie of bone, and the 

 serrated laminee of the crystalline lens in the eyes of birds and fishes. 



The eye-piece c is of use when it is requisite to investigate the in- 

 timate structure of delicate tissues ; and also in observations upon 

 fossil infusoria, volvox, scales from moths' wings, raphides, &c. The 

 employment of this eye-piece, when a higher power is required, obvi- 

 ates the necessity of using a deeper object-glass, which always occa- 

 sions a fresh arrangement of the illumination and focus. It must be 

 borne in mind, that the more powerful the eye-piece, the more apparent 

 will the imperfections of the object-glass become; hence less confidence 

 should be placed in the observations made under a powerful eye-piece 

 than when a similar degree of amplification is obtained with a shallow 

 one and a deeper object-glass. 



The degree of perfection in the construction of the optical part of a 



