56 THE MICROSCOPE. 



Inequalities amounting to '000002 line occnr, stripes of 

 another colour appear in them. 



Schmidt's goniometer positive eye-piece, for mea- 

 suring 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, care being taken that they lie perfectly, 

 fat, 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. 



Erector eye-pieces and erecting prisms are employed 

 for the purpose of making the image presented to the 

 eye correspond with the position of the object. They 

 are most useful for minute dissections, but the loss 

 of light occasioned by sending it through two addi- 

 tional surfaces is a drawback impairs the sharpness 

 of the image. Nachet designed an extremely ingenious 

 arrangement whereby the inverted image became erect; 

 he adapted a simple rectangular prism to the eye- 

 piece. The obliquity which a prism gives to the 

 visual rays, when the microscope is used in the erect 

 position, as for dissecting, is an advantage, as it brings 

 the image to the eye at an angle very nearly correspond- 

 ing to the inclined position in which the microscope is 

 ordinarily used. 



The Value of Eye-pieces. The magnifying power of 

 Ross's lowest eye-piece A is about 5; that of B, 8 to 10;, 

 C, 15; D, 20; and E, 25. 



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, 

 the circulation in the frog, etc., the A eye-piece is best 

 adapted. 



For the examination of details of minuter objects, 

 the B eye-piece is preferred; the pollen of flowers^ 



