Through the Looking Glass.
( With apologies to Lewis Carroll ).
By Michael Race, Australia
The simple brass drum microscope was a popular instrument for beginners and one which I enjoyed owning as a boy back in the mists of time. Then, much as now, I loved placing whatever would fit on the stage to see what it looked like in the finest detail despite having only one objective. As a result of this interest I remember being taken to see an elderly gentleman whose unusual profession was that of an ecclesiastical valuer, and whose hobby was Natural History. Living in a big white house in Buckinghamshire, this gentleman’s study was a large room filled with books, glass cases, stuffed animals, bits of plants and a large brass microscope on a very large desk in front of the bay window. Though somewhat gloomy with that dank smell of formalin the room was filled with the prospect of new discovery, and all with that splendid brass microscope, or so I imagined.
Like most folk I think, I am still attracted to those fine brass instruments which display such a variety of knobs,levers and draw tubes etc., indeed it was whilst admiring the display of microscopes on a particular website that I thought perhaps I could try using my microscope in the manner of a microscopist a century ago. The older microscopes exhibit a wonderful variety of design with many variations and improvements through the years on just about every component – all except one.
Almost without exception modern microscopes have in-built illumination as one of their essential features; the kind of thing one takes for granted these days. The lab bench does not have to be in front of a north facing window, and on a day of limited clemency the candles or oil lamp will not have to be trimmed every hour or so. We all probably get on with our observation of specimens without giving much thought to the light source as long as it accords with Mr. Köhler's findings.
As it happens my present microscopes date from the 1950s/60s and two of them do not have an inbuilt light source. There have been several instances when playing with various set-ups that I have found this to be a distinct advantage, especially when using simple arrangements with ultra violet and infra-red LEDs and therefore making use of the microscope mirror. Of course I do have, and regularly use, contemporary LED and tungsten light sources which connect beneath the sub-stage condenser, but it's the other occasions that may provoke some thought here.
I had been wondering what sort of problems the early microscopist might be faced with when having to depend upon the simple mirror and perhaps a less than reliable source of light, especially in the evenings. I mean how many of you, my readers, have actually used an instrument with a mirror? The sub-stage mirror appears to have come down to us from the very earliest times of microscope construction, and for the most part remained quite unchanged – unchanged that is for about 350 years ! ! Indeed economical student stands may still be purchased today with a mirror. And why not indeed? there are things yet to be learnt by the use of this simple device.
Robert Hooke (1635 – 1703) being a man of great enthusiasm and practical bent, did have a mirror on his microscope, and it looked pretty much like the one I have here, if the old drawings are to be believed. Further reading suggests that Descartes (1596 – 1650) who preceded Hooke, recommended the use of a mirror and a light gathering globe filled with water or oil. But as Descartes was more a man of fine theories, it is believed that Hooke would have been the first to actually construct and use any kind of mirror, and for that matter the light collector. I am unable to say whether these early mirrors were made of speculum metal or not, but it would seem that silvered glass was not uncommon in the 1650s, especially in smaller sizes. So perhaps the earliest microscope mirrors were indeed of glass construction, just as they still are today.
So in order to put the clock back, so to speak, I decided to remove the sub-stage condenser just to see what sort of images could be seen by relying wholly upon the mirror and a bowl of water as a collector for the light. The source of light and collector need to be close to the microscope in order to avoid too much loss of light, and because I normally use a universal (Zernike) condenser which occupies most of the sub-stage space it just had to be removed.
For my first experiment I decided to use a wine glass filled with water; doubtless gin or vodka would serve just as well but Mr. Hooke makes no mention of these liquids. My wine glass produced a focal length far too short to clear the microscope stage so something of larger diameter was required. In the absence of gold fish with bowl further thought was needed, so a visit to the local second hand shop produced a very nice thin glass bowl of quite modest price and some four inches in diameter. This gave a more workable focal length, and so was adopted for the experiment. In his Micrographia, Hooke claimed he used brine instead of water since there would be an advantage with the higher refractive index and inhibit algal growth. I didn't think I would use the brine as my set-up is unlikely to be in use long enough for any algae to grow, and the higher refractive index could be more of a hindrance.
First of all my north facing window will have to be west because my study window has that aspect, but setting the 'scope up for daylight certainly gives excellent illumination especially if there is a blue sky with a few fluffy white clouds. And no wires draped across the bench! The mirror is of course turned in order to use the concave side, which will focus the light directly onto the underside of the slide. The resultant image was remarkably evenly lit and well defined. I could easily use all objectives up to an including the X40, but did not bother try the oil X100: doubtless it would work just as well. Of course without a diaphragm, some judicious fiddling with the mirror helped improve contrast, but without a sub-stage condenser the microscope still performs awfully well, especially considering Ernst Abbé's design for a condenser didn't appear until around 1870.
The real test now came in the evening when I would need to step a long way back from the present use of LEDs. Oil and gas were long used as microscope light sources so without these I had to try a candle. The flame is small, not terribly bright and moves down as the wax burns away, but by adopting my nineteenth century state of mind (comments are not required here!), these problems would neither deter nor prevail. The glass bowl was filled with water and placed upon a book in order to adjust the height. With a new candle set in the holder, we need everything in a straight line from the centre of the flame, through the middle of the bowl of water to the centre of the microscope mirror. Here I used a piece of white card across the mirror in order to see the image of the flame cast through the bowl and to adjust it as needed.
Using daylight seemed to have very little disadvantage, certainly at the lower powers, the change to candle power proved to be somewhat drastic and most limiting. Apart from low brilliance the flame is very yellow and thus makes viewing stained or prepared slides quite difficult. The microscope camera produced very orange images not easily corrected in Photoshop. Up to powers of X200 the 'scope remains quite usable, but it pays to get the eyes dark adapted and work slowly.
Oil lamps used for microscopy appeared to have had a wide wick and enhanced air supply to the base of the flame, thus providing a large area of illumination along with less red or yellow light, albeit never truly white. Gas burnt in a mantle, on the other hand does burn bright white, so the gentlemen microscopist with these lighting services may well have faired much better than the commoner burning his candles and rush lights, probably made by the good wife.
In conclusion, I found that the bowl of water as light collector made a considerable contribution to light gathering, but the stack of books upon which it is placed, really should be of an appropriate subject. In day light I used the set-up both with and without the condenser with good results, however, I seriously think the mirror still has a part to play in amateur microscopy especially as it can be moved around and placed almost anywhere without power cords. In good daylight the mirror may well yet remain unsurpassed serving both the squire and artisan equally well.
Comments to the author are welcomed.
© Microscopy UK or their contributors.
Published in the July 2011 edition of Micscape Magazine.
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