242 Transactions of the Society. 



V. — TJie Working Aperture. 

 By Edward M. Nelson. 



{Read 20th March, 1901.) 



It will be generally recognised that it is important to distinguish more 

 accurately than hitherto has been the practice, the precise ratio of 

 the diameter of that part of the objective which is utilised to the 

 diameter of the lens itself ; in other words, vague generalities such 

 as " about f cone," " about § cone," &c, are now not sufficiently 

 •explicit. A microscopist has only to work a short time with large 

 axial cone illumination before he will become aware how great a 

 difference in the image is produced by a trifling alteration in the size 

 of the illuminating cone. 



It is obvious, therefore, that in recording delicate observations with 

 the Microscope it is advisable that the precise ratio of the utilised 

 diameter of the objective should be stated. 



Hitherto it has been customary to record the N. A. of the objective 

 on the nose-piece ; but this, in many instances, is not only a useless 

 piece of information, it is often actually misleading. The information 

 really required is the amount of the working aperture actually utilised 

 during an observation, and not the total amount of the aperture avail- 

 able for utilisation. Similarly, a record of the actual weight of a body 

 would be more important than the information that it was weighed in 

 a balance capable of weighing 1000 grains. There are numerous re- 

 corded observations with oil-immersion ^ 2 °f 1'3 N.A., where the 

 aperture utilised has been less than ■ 3 N. A. ; hence these records, to 

 say the least of them, are misleading ; an ordinary i-in. objective 

 with a | cone would probably have given a better and more truthful 

 picture. It is a dangerous practice to use an objective of large aperture 

 when only a small portion of its available aperture is employed ; for 

 this is one of the most ready means of producing false images by the 

 doubling of structure, or by the insertion of false intercostals, &c. 

 Now, as the golden rule for obtaining truthful images is to employ as 

 large a proportion of the total aperture as possible, it becomes a matter 

 ot importance that the amount of that proportion should be known. 



It is of little avail to say that a great many histological objects 

 do not contain periodic structures, and therefore do not lend them- 

 selves to the manufacture of false images ; because, however elementary 

 the object may be, its image will always be improved when the objective 

 employed has a large proportion of its total aperture utilised. Further, 

 microscopists, as a class, are accustomed to use powers higher than are 

 necessary ; for example, an oil-immersion ^ w ^ n a small cone is 



