i6 



MICROSCOPE AND ACCESSORIES. 



[CH. I. 



§ 28. Aperture of Objectives. — The angular 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 

 [i. e., 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 the original table or with that in 

 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 recommendaiions of the committee, published in the Proceedings, Vol. XII, 

 p. 250, are as follows : 



" Believing ill 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 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 



f """ J" "V' as defined in section 1 — Be, for the short 



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

 es, and 216 mm., or S}4 inches, for the 

 long tube, 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 

 with that of the upper end of the tube. 



4. That the mounting of all objectives 

 of 6 mm. {% inch) and shorter focus 

 should be such as to bring the optical 

 center of the objective 1% 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 y 1 ^ to ^ 

 mm. ( iTj, to T j S inch) in thickness. 



These recommendations give a distance 



Fig. 25. The tube of a microscope with of ro inches (254 mm.) between the par- 



ocular micrometer and nose piece in focal plane of the ocu]ar and the opt i ca i 



position to show that in measuring cent er of the objective for the long tube, 



tube-length one must measure from an ,i are essentially in accord with the 



the eye lens to the place where the ob- actua] p ractice of opticians. 



jective is attached. (Zeiss' Catalog, At the request of the committee, a ioint 



3°>- 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 and a willingness to 



adopt them." 



(Signed) Simon H. Gage, 



A. Clifford Mercer, 

 Chari.es E. Barr. 



