592 Dr. G. Johnstone Stoney on 



on the hypothesis that the objective has been accurately 

 corrected for aplanatism there is only one distance of object, 

 with its corresponding one distance of the image on the other 

 side of the objective, for which the aplanatic correction can 

 be satisfactorily made, and it is only when object and image 

 are at these unique distances from the objective that the 

 light from all the annuli of the concentration image can 

 concur to reach the same focus. This adjustment is known 

 to be one which has to be made with extreme care to bring 

 out the fall efficiency of a good objective of large NA. 



44. All this is well known to microscopists : but it is not 

 certain that they always equally appreciate that the same 

 care must be bestowed on adjusting condensers of large 

 aperture in order to secure the best, or even good results. 



What adds very much to the usefulness of image x, the 

 concentration image, is 1°, that it can be made to exhibit 

 na — i. e. the numerical aperture of the condenser ; 2°, that 

 it supplies useful information as to the light transmitted from 

 the condenser to the microscopical object ; and 3°, that by 

 its help the important adjustment of the distance of the source 

 of light from the condenser which is above referred to, can 

 be made. When this adjustment has been effected the light 

 which comes through a minute hole in a screen placed in 

 front of the flame, or other source of light, will produce 

 light in the concentration image which will extend in that 

 image out to the dotted circle with radius na, as estimated on 

 the NA scale, if the microscope has been armed with an 

 objective of as large or larger aperture. If the NA of the 

 objective is less than the na of the condenser, then the size 

 of the concentration image, the visible part of the indicator 

 diagram, is too small to show the whole of the disk of light 

 which the condenser is competent to produce from each 

 punctum of the flame. Accordingly, in order that we may 

 be able to avail ourselves of the full aperture of a wide 

 angled condenser the adjustment referred to above must be 

 made with care, and in fact quite a moderate departure 

 from it diminishes in a considerable degree the efficiency of 

 the condenser. 



45. Before proceeding to make use of the concentration 

 image in carrying out the experiments which will occupy 

 our attention in the next part of this series of papers, it 

 appears desirable to call attention to the circumstance that 

 although the concentration image as seen on looking down 

 the tube of the microscope is a good, and even a very good, 

 representation of the central part of the indicator diagram, 

 it is not an exact reproduction of it. This is partly on 



