The Working Aperture. By Edward M. Nelson. 243 



used instead of a \ with a large cone, and so also is a ^ used in place 

 of a ^-in. It would add importance and strength to the truthful- 

 ness of a picture if it were accompanied by a statement of the precise 

 ratio of the utilised aperture to the total aperture of the lens through 

 which it was drawn. Many ways in which this might he carried out 

 will readily suggest themselves ; thus the ratio might be called the 

 working ratio, and be represented by the letters W.R. For example, 

 under a drawing might be written, apo. £ N.A. • 9;"j, W.R. 0*75, 

 which would mean that an apochromatic |-in. objective of 0"95 N.A. 

 illuminated by a f cone of axial illumination was used. 



If this fraction should appear less than 0*5, all intelligent micro- 

 scopists will know what value should be accorded to the picture. For 

 example, the well-known Zeiss photograph f of Pleurosigma angu- 

 lation, taken with the view of confirming Prof. Abbe's diffraction 

 image theory, would come out like this : — P. angidatum, apochromatio 

 T V 1-3 N.A. 0-23 W.R. 



Hitherto the measurements of this working ratio have been merely 

 rough estimations by the unaided eye; it may be as well now to con- 

 sider some alternative and more accurate method. One of the simplest 

 is the measurement of the Ramsden disc in front of the eye-lens of 

 the eye-piece by means of a magnifying-glass and a micrometer. It 

 should be borne in mind that the size of the disc is inversely propor- 

 tional to the power of the eye-piece ; i.e. the lower the power of the 

 eye-piece the larger will be the disc. For this purpose a divided lens 

 dynamometer, such as is used with telescopes, would answer admirably ; 

 but then this piece of apparatus is somewhat expensive, and is hardly 

 a part of a microscopist's outfit. 



As a guide to the size of the images obtained by this means, an 

 apochromatic ^ with a No. 2 compensating eye-piece gives a disc 

 whose diameter is about -f s in. Another simple and efficient method is 

 to use the draw-tube as a second Microscope, and fit an adapter at the 

 bottom to carry a low-power objective, the eye- piece being an ordinary 

 micrometer eye-piece, having a graduated scale in the plane of its 

 diaphragm. The Zeiss large a* is a convenient objective to use for 

 this purpose, as its power can be readily altered by merely turning 

 the adjustment collar. Perhaps it is better that this second Micro- 

 scope should be mounted separately with a tube to itself, for then it 

 can be inserted into the Microscope, like an ordinary eye-piece, with- 

 out disturbing any of the adjustments. 



This secondary Microscope is for the purpose of magnifying the 

 back of the objective, just as if it were an object upon the stage. The 

 size of the central illuminated disc is measured, and this is divided 

 by the size of the diameter of the total aperture, the quotient being 

 the Working Ratio or W.R. It will be noticed that it is unnecessary 

 to convert the micrometer readings into actual measurements in either 

 inches or mm., as only a ratio is required to be determined ; therefore 



t^This photograph has lately been withdrawn. 



