244 Transactions of the Society. 



one need not proceed farther than to read the number of the divisions 



on the micrometer scale, which may be a purely arbitrary one. For 



example, let the total aperture equal 18 divisions, and the utilised 



13 

 aperture 13, then j s = 0'72 W.K. (fig. 38). 



The N.A. of the central illuminated portion can be readily found 

 from the W.K. by multiplying the W.R. by the total N.A. of the 

 objective ; this might be called the Working Aper- 

 Fig. 38. ture, or W.A. 



if the W.R. is written after a lens, it is better to 

 supplement this information with the N.A. as well, 

 because a W.E. of say 0*75 with a half-inch of 

 0'3 N.A. means something very different from the 

 same W.R. of "75 with an apochromatic 12 mm. of 

 • 65 N.A. In the first case the W.A. is only ■ 23, 

 but in the second it is 0*49, or more than twice as 

 much. Some microscopists, in recording their work, may perhaps 

 prefer to give only the W.A., while others will give both the W.R. 

 and the N.A. of the objective they employ. 



When objectives are being compared, it is of the greatest import- 

 ance that the W.R. should be identically the same in each case ; if not, 

 the objective that was used with the larger W.R. would be much 

 handicapped in the trial. 

 Summary : — 



The W.R. is the diameter of the disc ot light seen at the back ot 

 an objective (the source of light being accurately focussed by the sub- 

 stage condenser upon the object) divided by the diameter of the back 

 lens * of the objective. 



The W.A. is the W.R. multiplied by the N.A. 

 If it is required to set off with a given objective a given W.A., it 

 is only necessary to divide the required W.A. by the N.A. of the 

 objective and then multiply the quotient by the diameter of the back 

 lens. Example : — Required the diameter of the bright disc in order 

 that an apochromatic 4 mm. of 0*96 N.A. may have a working aper- 

 ture of " 6, the diameter of the back lens being 18 divisions of the 

 micrometer. Then, 



0-0 



0-96 



= 0-625; 0-625 x 18 = 11-25. 



Therefore if the iris of the substage condenser is adjusted so that it 

 will give a diameter to the bright disc of 11 "25, then the objective 

 will have • 6 W.A. 



Most microscopists are painfully aware, after they have caught 



* Provided that the diameter of the back lens is not larger thin that required 

 by the full aperture of the objective. The correct diameter of the back lens is 

 found by multiplying twice the N.A. by the focus. 



