During the intervening period since the last article (13th March 2012) I realised that though the 'seamless condenser' setup I'd finally settled on entertained me for several weeks, it slowly became apparent that I wasn't utilising the BF facility a great deal. And so it occurred to me that by not having a BF facility I could plant a permanent circular opaque stop of appropriate diameter on the field lens which would cater for the lower objective range more effectively than I had already been able to achieve with the single polar stop size . Thus the smaller permanent opaque stop would suffice for the x3,x6 and x 10 objectives, and an overlapping polar disc would, with its crossed polar companion, broaden the cone that was necessary to satisfy the x20 and x40 objectives. So by foregoing BF I broadened the observational base in DF and COL. For the record the small permanent stop would not only provide DF and COL for the x3,x6 and x10 objectives, it would allow a fairly good performance in BF when imaging with the x20 and x40 objectives, since the small obstruction from the permanent stop would be insignificant. In short I opted for this more complex arrangement which is illustrated below. I only referred to in the last article as a possible option :-

The Case for an Iris Diaphragm for COL Illumination

During this period of putting the 'MK11b'condenser through its paces I was often subconsciously stopping down the BF iris diaphragm when using COL, a dyed in the wool habit acquired from days of old when using BF. On one particular occasion, when employing the x40 objective with COL, I noticed a distinctly improved image when observing a prepared slide on which the specimen had been embedded in a slightly thick mountant setting. In short, it seemed that the slight iris closure which masked the outer fringe of the illuminant's cone diminished the resulting effects of spherical aberration induced by the overly thick preparation rather impressively. Further tests confirmed that this was a technique that also worked well with the x20 objective, though the need to do so with this objective was infrequent because of its relative insensitivity to mountant thickness. Of course it follows logically that if the outer margin of the COL cone is restricted somewhat by the iris, then the inner margin of the stop must be of such size as to maintain the annular ring of light, which the subtle adjustment of the condenser's elevation level will achieve rapidly. The overall effect is similar to the 'focusing' action of Leitz's Heine condenser........ though the latter is much quicker in action, it doesn't have the virtue of being able to modify the actual annulus's inner and outer diameter independently.

The diagram above also illustrates the placement of a more effective iris diaphragm below stage. The standard iris in most substage condensers is poorly placed for constricting the emerging COL cone of light from the lamphouse as it would eventually form a shadow in the field's central zone. Ideally it needs to be in the same plane as the polar disc assembly (Field lens), so that the tendency to eventually form shadows in the central imaging zone is reduced significantly, and also the diffraction generated by both the iris diaphragm and polar/stops originates from the same plane. This incidentally aids imaging clarity through the phase telescope, as both the iris and polar stops will appear sharply defined in its field of view.

The Effects of Iris Closure on the Ascending Cone of Illumination

Gaining control of the COL 'light tube' brings about visual improvements that can augur well with pond life observations, since the latter rarely offers perfect optical viewing conditions. The effective coverslip/water thickness in these situations often exceeds the ideal, and since COL illumination is rather sensitive to overly thick preparations, muting the NA by restricting the width of the outer illumination cone often results in an improved image. However the application of iris closure needs to be subtle in most cases, and is only effective with the higher power dry objectives whose sensitivity to preparation thickness is most acute. The size of the inner edge of the annulus also effects an objective's performance especially through thickish mountants.

Of course the annulus can be offset to raise oblique illumination effects too, by lateral shifting of the iris.

Since the central stop's apparent diameter can  be modified by condenser elevation changes, and the iris's masking capability makes for a variable annulus to suit both the preparation and the eye. If there is an inherent flaw here, then I suspect some observers might find that the almost infinite range of settings a little daunting to absorb into the psyche. But like all new skills, time gradually brings about familiarity with the task, and after several sessions of usage have elapsed it becomes far easier to cope with.

The accuracy of the microscope's optical alignment in this venture cannot be over emphasised. The extra parts that are included along the optical train below stage need to be truly concentric with the optical axis of the 'scope. Adapting, if necessary the iris and condenser so that they are adjustable via the traditional 3 x 120 degree screw system is well worth the effort in the long run. Ideally the iris and condenser need to be independently centred for best results. Once accurately aligned, the phase telescope can be dispensed with, leaving uninterrupted lighting variations to be raised to suit the specimen.

Concluding Thoughts

I now have a much more useful and practical substage assembly whose light output can be subtly tweaked to raise DF, and COL, AND virtually nullify the latter's tendency to get upset in 'deep' water observations with the x40 objective......... to an impressive degree, .....................and all accomplished without losing sight of the quarry. True, the use of the iris is infrequent, but its there to be used, albeit subtlety, if and when necessary. The loss of pure BF isn't an issue for me since with DF and COL I am able to perceive the various specimens in superior lighting with higher contrast anyway. So the vast majority of aqueous/pond life preparations I observe utilise the DF and COL almost exclusively. Throughout the duration of the observing session I vary the elevation control of the condenser above and below the DF/COL's demarcation position frequently for a given objective, then leaving it set to suit that particular specimen. The polar companion filter gets a twist now and then when I want to change from the lower objectives to the x20 x40 battery, and vice versa.

Though the reader might understandably feel that getting rid of BF to be unwise, it is a simple fact that the imagery generated with the small opaque stop when employing the mid to higher power objectives, is in fact so very nearly identical to pure BF anyway, because only a small central obstruction is missing from the raised imagery. The only difference between this and pure unadulterated BF is a very slight increase in contrast with the small stop.

I have now settled into a very satisfying practice of observational technique, which has slowly become less complex than initially experienced. Frankly speaking, I wouldn't swap my modified substage condenser assembly with anything else : so satisfying and convenient I find it. It is also useful when videoing aquatic creatures because the lighting variations enhance the visual perception of form and motion, which a conventional fixed illumination setting cannot satisfyingly compete with.

Finally ........

We've lapsed into the 21st century by 12 years, and so far there's no sign of a major shakeup below stage to combine the lighting disciplines into a compact rapidly adjusted piece of kit since Köhler's revelation, and even longer ago when the similar Critical illumination was instituted ..................... That's an amazing 120 years or more. The mind boggles at all the changes across the huge spectrum of human industry in the same span of time. In the emerging professional microscopy practice during that period, the stand has been endowed with parfocality, parcentricity, infinity optics, DIC, and Phase contrast, to name some of the major advances.

Yet in some ways the compound microscope remains a relic of former times. It's still the image generator of old, though many amateurs would defend this state of affairs, especially as often as not the more modern stands relinquish the elegance that the earlier breed so obviously displayed. I still feel that there is a case for the evolution of a hybrid 'scope that has all the virtues of grace, form and function of former stands, but under the bonnet the substage sports a far more efficient condenser of lighting variations, that can be implemented with subsecond rapidity. In one way this has happened in principal in mainstream photography. The outer casing and shape of a modern digital camera is not that far removed from former film examples, but inside they are a different breed altogether, where efficiency and processing speed exist at astonishing levels. This cheap high speed imaging facility on digital cameras means we can abandon the old inhibitions of wasting costly colour film, and enjoy instead digital's liberating advantages.

I genuinely find my ad hoc seamless substage condenser truly liberating, despite its minor imperfections. During an observation session I continually apply changes to the illumination of living micro life forms as the seconds pass, and then settle on a setting which favours a particular critter. I can check at any time in a second or two whether another specimen deserves slightly different lighting etc..

Now the cherry on the cake would be to have an image recording system that captured these scenes immaculately and easily, either in still or video mode. Capturing imagery onto the CMOS sensor has proved very satisfying with my DIY photomacroscope, but I've yet to come across a truly flatfield image projection system in compound photomicroscopy which does the business. Maybe I'll get lucky again and stumble upon another ad hoc optical combination which does justice to the image by sending it to the CMOS sensor as the eye sees it?

Whilst I understand the sentiments and the science behind the practices of brightfield illumination, it seems to me that many aspire to achieve the ultimate in the resolution capability of the compound microscope as a means to an end. As a consequence there has been far too much adherence to 'correct procedural' matters in illumination techniques, and inevitably the amateurs' microscope has remained rather stunted.

The pedantic illumination system in brightfield microscopy is in my view is a bit like having to step out of your car with a spanner to change the headlights from full beam to dip, and visa versa? The fact is that, in some disciplines, microscopy is a mile behind the times. The substage in particular, from my perspective is like an old fossil : a remnant of the past, and has remained unchanged to this very day. So it seems that the modern amateur's microscope hasn't fully emerged from its Chrysalis and spread its wings properly!