CH. /] 



MICROSCOPE AND ACCESSORIES 



30. Aperture of Objectives. The angular aperture or angle 

 of aperture of an objective is the "angle contained, in each case, be- 

 tween the most diverging of the rays issuing from the axial point of an 

 object [/. ., a point in the object situated on the optic axis of the 

 microscope], that can enter the objective and take part in the formation 

 of an image. ' ' (Carpenter) . 



in the direction of uniformity and in general in the direction recommended by the 

 writer and Mr. Bausch and the committee of the American Microscopical Society. 

 The recommendations of the committee, published in the Proceedings, Vol. XII., 

 p. 250, are as follows : 



"Believing in the desirability of a uniform tube-length for microscopes, we 

 unanimously recommend : i. That the parts of the microscope included in the 

 tube-length should be the same by all opticians, and that the parts included should 

 be those between the upper end of the tube where the ocular is inserted and the 

 lower end of the tube where the objective is inserted. 



2. That the actual extent of tube 

 length as denned in section i Be, for the 

 short or continental tube, 160 mm., or 6.3 

 inches, and 216 mm., or 8^ inches, for 

 the long tube, and that the draw tube of 

 the microscope possess two special 

 marks indicating these standard lengths. 



3. That oculars be made par-focal, 

 and that the par-focal plane be coincident 

 with that of the upper end of the tube. 



4. That the mounting of all object- 

 ives of 6 mm. (% inch) and shorter focus 

 should be such as to bring the optical 

 center of the objective \% inches below 

 the shoulder, and that all objectives be 

 marked with the tube-length for which 

 they are corrected. 



5. That non-adjustable objectives be 

 corrected for cover-glass from ^ to T 2 

 mm. ( I y^ to T |^ inch) in thickness. 



These recommendations give a dis- 

 tance of 10 inches (254 mm.) between the 

 par-focal plane of the ocular and the op- 

 tical center of the objective for the long 

 tube, and are essentially in accord with 

 the actual practice of opticians. 



At the request of the committee, a 

 joint conference was held with the opti- 

 cians belonging to the Society and present at the meeting. They expressed their 

 belief in the entire practicability of the above recommendations and a willingness 

 to adopt them." 



(Signed) SIMON H. GAGE, 



A. CLIFFORD MERCER, 

 CHARLES E. BARR. 



FIG. 25. The tube of a microscope with 

 ocular micrometer and nose piece in 

 position to show that in measuring 

 tube-length one must measure from 

 the eye lens to the place where the ob- 

 jective is attached. (Zeiss 1 Catalog.} 



