64 NOTES ON MICROSCOPICAL OPTICS: 



A great and very serious defect in the construction of nearly all 

 condensers of the present day, with the exception of the modest 

 ''Abbe " Condenser of two simple uncorrected lenses, is that the Iris 

 and the ring for dark ground-stops are placed too far from the back 

 lens instead of being close to the anterior focal plane of the con- 

 denser. It is easily shown that such a remote Iris-opening or dark 

 ground-stop produces decidedly oblique illumination of the extra- 

 axial points of the object. With direct light this leads to an 

 undesirable variation of the type of image and of resolving power 

 in different parts of the field. With dark ground illumination the 

 result is even more serious, for it is then necessary to use a far 

 larger central stop to secure a dark background over the whole field 

 than would suffice if the stop were placed close to the anterior focal 

 plane of the condenser : such an unnecessarily large stop is highly 

 objectionable, because it reduces the visibility of the coarser struc- 

 tures in the object. 



The increasingly bad position of the iris in the condensers of 

 higher power and shorter focal length supplies practically the whole 

 explanation of the universal experience that high-power condensers 

 will not work satisfactorily with low power objectives, especially for 

 dark ground illumination. 



The great thickness of the mechanical stage in English stands 

 of the highest quality is the chief reason why the iris and " turn- 

 out-ring " of high-power condensers have to be mounted so far 

 below the back lens and a profound modification of the design of 

 the stage with a view to making the part projecting over the con- 

 denser as thin as possible therefore appears to be the most desirable 

 improvement of microscope stands from the optical designer's point 

 of view. 



In concluding these remarks on the optical design- of microscope 

 lenses I wish to point out that the whole subject is adequately dealt 

 with in my lectures and classes at the Imperial College, and that 

 students attending these for two or three years will be turned into 

 competerit designers, provided that they have a liking and natural 

 aptitude for applied mathematics, are good at numerical calculations, 

 and of an inventive type of mind. 



As regards the (icfiidl iiuihiiKj of iniriosropc object ires, it must 

 be borne in mind that the excellence of a computed lens system may 

 be completely swamped by comparatively slight imperfections of 

 workmanship, and that high accuracy in this respect is therefore of 

 the utmost importance. In lenses of high N.A. computation shows 

 that a departure from the prescribed radii and thicknesses by a 

 fraction of a thousandth of an inch may lead to a notable loss of 

 perfection, and the polished surfaces must also be truly spherical 

 within less than half a wave-length of light. These limits can be 

 easily observed if modern methods of gauging and measuring are 

 adopted, and if all surfaces are polished to accurately made and 

 conscientiously used test-])lates. In the later years of my connection 

 with the optical industry quite large batches of lenses used to be 

 made directly from purely theoretical calculations of objectives of 

 new types without any preliminary trials and without any experi- 

 mental changes in the finished objectives, 95 or more per cent, of 

 which would be found satisfactory in all respects just as they came 



