298 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



si»iral spring) and acting on tlie slide A by means of the two small 

 revolving disks attached loosely by a triangular fitting to the pinion 

 at E. By making the slope of the wedge very acute, and the thread 

 of the screw very fine, the focussing movement is rendered unusually 

 delicate. This latter form of fine adjustment is designed to be applied 

 to more exjiensive Microscojjes. 



Abbe's Stereoscopic Eye-pieee.* — Fig. 45 represents this instru- 

 ment in section. The body A A' contains three prisms of crown 

 glass, a, h, and h'. The two eye-pieces B B' are let into the top plate, 

 the former being fixed, whilst the latter has a lateral sliding move- 

 ment ; the bottom plate carries the tube C for inserting the eye-piece 

 into the microscope-tube like an ordinary eye-piece. 



Fig. 45. 



A 



The two prisms a and h are united so as to form a thick plate with 

 parallel sides, their continuity, however, being broken by an exceedingly 

 thin stratum of air — less than 0"01 mm. — inclined to the axis at an 

 angle of 38 • 5°. The cone of rays from the objective is divided into two 

 parts, one being transmitted and the other reflected. The transmitted 

 rays pass through a, b without deviation, and form an image of the 

 object in the axial eye-piece B. The rays reflected at the angle 

 shown in the figure pass through the second surface of the prism b 

 (upon which they are incident at right angles), and emerging at an 

 inclination of 13° with the horizontal, are totally reflected into the 

 eye-piece B' at an angle of 90° by the hypothenuse surface of the 

 right-angled equilateral prism h', the axis of which also makes an 

 angle of 13° with the axis of the Microscope. 



* Zeitsohr. f. Mikr., ii. (1880). 



