i6 



MICROSCOPE AND ACCESSORIES. 



\CH. I. 



§ 28. Aperture of Objectives. — Th 



jular aperture or angle of 



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

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

 [z. i?., a point in the object situated on the extended optic axis of the 

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

 of an image." (Carpenter). 



43). If the table in this edition is compared with ihe original table or with that ill 

 the previous edition of this book some differences will be noted, the changes being 

 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: 1. That the parts of the microscope included in the 

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

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

 lower end of the tube where the objective is inserted. 



2 Thai the actual extent of tube length 

 as defined in section [ — Be, for the short 

 or continental tube, 160 mm., or 6.3 inch- 

 es, and 216 mm., or 8 y z inches, for the 

 long lube, and that the draw tube of the 

 microscope possess two special marks in- 

 dicating these standard lengths. 



3. That oculars be made par-focal, and 

 that the par-focal plane be coincident 

 wilh that of the upper end of the tube. 



4. That the mounting of all objectives 

 ol 6 mm. (% inch) and shorter focus 

 should be such as to bring the optical 

 center of the objective ij 2 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 T ' (l \ f to $,% 

 mm. ( T l - to jljj inch) in thickness. 



These recommendations give a distance 

 of 10 inches (254 mm. ) between the par- 

 focal plane of the ocular and the opitical 

 center of the objective for the long tube, 



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 aud are essent i a lly in accord with the 



actual practice of opticians. 



At the request of the committee, a joint 

 conference was held with the opticians 

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

 in the entire practicability of the above recommendations aud a willingness to 

 adopt them." 



(Signed) Simon H. Gage, 



A. Clifford Mercer, 

 Charles E. Barr. 



the eye lens to the place where the ob- 

 jective is attached. (Zeiss' Catalog, 

 No. 30). 



