by Roy Winsby

A desire to draw.
Occasionally we may want to draw something we see through the microscope and there are two ways of doing this, the Camera Lucida and the Squared Graticule, which are explained as follows:-

The Camera lucida.
The Camera lucida does not have anything to do with a camera, but that is what it has always been known as, and it is an apparatus very popular in Victorian times before the development of a photographic camera which could be affixed to a microscope. I doubt if any Camera lucidas have been made for many years, though there must still be a good number of the older ones about as they were made by such well known makers as Zeiss Abbe, Leitz, Watson, Beck, that I know of, and no doubt many other makers made them too. I have never seen a Camera lucida advertised in dealers' catalogues issued in recent years, nor can I recall having seen one offered for sale, or even as an item wanted, in Sales & Wants lists until the recent issue of the Quekett Bulletin, Summer 1995 issue, which contained on page 30 a Wants ad for an original pattern Zeiss-Abbe camera lucida without filters.

Books on microscopy from around 1855 mention the use of photography as an alternative to the use of the Camera lucida, but with the later improvements in the photographic camera with its ease of an instant picture the purpose of the Camera lucida was superseded and brought about its demise. However, no matter how good and labour saving that modern appliances are, there is sometimes some aspect that is not quite as good as the old thing, and this is to some extent true in this instance. Whilst photographs are so much easier and quicker in their taking, the Camera lucida has one important advantage over the photographic camera, i.e., drawings can be made in depth.

We all know about the depth of focus, or, as some may prefer to call it, the depth of field, but whichever way you wish to call it, it is a fact that the higher the numerical aperture of the objective the more shallow will be the depth of focus. This means that, when using a high power objective, even small items have to be viewed at different levels of focus, and this is the drawback of the camera as it can only take a picture of any one focal plane. The Camera lucida does not have this problem, as drawings can be done in depth, the different focal planes being brought into view by a slight change of the microscope focus as the drawing proceeds.

A typical Camera lucida.
Camera lucidas have a lift-up or slide-sideways top lens unit which at first glance looks like the eye lens of an eyepiece, but it is more complex than that, for below it is a small prism with a half silvered surface which (as explained by Conrad Beck in a book entitled "The Microscope" published in 1938) transmits half the light upwards to the eye from the microscope and reflects by means of an adjustable mirror on an arm, about 6" to one side, the light from a sheet of drawing paper placed on the table at one side of the microscope immediately below the mirror.

The mirror reflects the image of the sheet of drawing paper through a small hole in the side of the Camera lucida eyepiece cap to the small prism, whereupon the eye sees the two images (the specimen and the drawing paper with the pencil) superimposed one on the other.

Camera lucidas appear to have been made for right handed people. In past years before we were more enlightened, right handed writing and drawing were the rule in schools, and left handed people had a difficult time. If you are left handed and not ambidextrous, then a Camera lucida is not for you, in which case go to Part 2 of this article dealing with drawing through the microscope using an eyepiece graticule.

How to use a Camera lucida.
How the Camera lucida is used is that looking through the microscope you see the tip of the pencil superimposed over the image of the specimen and you draw over the outline of the image. This, however, is not anything like as easy as it sounds, and can in fact be difficult and frustrating; some people are able to make perfect drawings of all kinds of specimens, whilst others find it difficult to use and never master it. Some Victorians must have mastered its use because many of the drawings of specimens through the microscope seen in old books were drawn using a Camera lucida.

Very few readers of this Newsletter will have a Camera lucida, and indeed not many will have actually even seen one, but as it forms an important part of microscopical history you may like to know a little more about it and how it works. You may, you never know your luck, possibly pick one up sometime, and you can always keep this article in mind should the occasion arise when you might unexpectedly obtain a Camera lucida. The microscope to be used need not necessarily be monocular but it should preferably have an upright eye tube or eye tubes, though only one eye tube is used. However, I understand there is another, rarer, form of Camera lucida made for use with the microscope in a horizontal position, which has a somewhat similar prism and does not require a mirror. I have never seen this latter type and doubt if many of you will ever see one anyway.

So, as regards using the conventional Camera lucida, set up the microscope in the normal way with a correctly illuminated and focused specimen using say a x10 objective. Temporarily remove the eyepiece and clamp the Camera lucida to the top of the eye tube, then replace the eyepiece. At this stage the Camera lucida top lens is lifted or swung sideways out of the way depending on its type.

At the side of the microscope place a sheet of white drawing paper with a pencil on, lying beneath the mirror. Temporarily place a small piece of paper on the stage covering the specimen, so as to block out the image, and bring the Camera lucida top lens into position and look through. The paper at the side of the microscope with the pencil resting on should be seen clearly. If not, adjust the swivel mirror at the end of the arm until the pencil can be seen central on the paper. Remove the paper off the top of the slide and look through what will be seen as a small bright centre part of the Camera lucida top lens - it is in this bright centre part that the image appears. There is no need to refocus the microscope unless you change the magnification. Looking through the Camera lucida top lens you should see the image superimposed on the drawing paper and, in theory, you simply pick up the pencil and draw round the image - if you can at first see both image and pencil clearly then you are indeed lucky.

Getting the contrast right.
Usually beneath the Camera lucida top lens is a wheel with a few small supplementary glasses which are neutral density filters varying in density, the purpose of which is to vary the relative brilliance as between the specimen and the drawing paper. Looking through the Camera lucida top lens turn the supplementary filters wheel to each of these small filters in turn, which may bring up the image of the specimen nice and clear, but not showing the pencil, or it may show the pencil and paper but no image of the specimen - you have to find the happy in-between contrast which will show both image and pencil, though both image and pencil will not now be at their brightest since you are obtaining the best of both working together. Some Camera lucidas have a further one or perhaps two supplementary filters fastened to the side of the eyecup which can be swung up into position to give a little more contrast.

Balancing the light.
Having gone through all that procedure, some people still cannot find the necessary happy in-between contrast, and here is the secret ingredient - you have to balance the light intensity as between the illumination of the specimen seen through the microscope and the light from the table lamp shining on the paper. The microscope illumination is the prerequisite and is set as standard, it is the light from the table lamp shining on the paper which has to be controlled and the most simple thing for this is a Woolworths' standard light dimmer switch fixed between the power point and the lamp.

A few tips!
So, having got set up, looking through the microscope you draw the pencil over the image seen, but of course the actual drawing you are doing is the size of the magnified image, i.e., using a x5 eyepiece and a x10 objective, the drawn image will be about 50 times the size of the image on the slide depending on the position of the drawing paper. When holding the top lens, particularly when turning the filters wheel, keep your fingers out of the way of the small hole immediately below and to one side of the Camera lucida top lens which faces the mirror - this is the opening to the tiny reflecting prism in the Camera lucida head.

The Camera lucida can be used with a microscope having an inclined eye tube, but two difficulties arise. One is that unless the clamp is securely fixed to the eye tube, the mirror at the end of the arm swings down with gravity and you have to hold it up. The other is that the sheet of drawing paper needs to be on a board sloping at the same angle as the mirror and whilst it is more comfortable to use an inclined eye tube I think it simpler to use a vertical eye tube microscope. If you have a modern made microscope where accessories are still available for it such as the Russian Biolam range, you should find that vertical monocular eye tubes are available for purchase.

The Graticule.
The graticule, also known an an eyepiece micrometer, is a small glass disk etched with a squared grid for area measurement, or a scale for linear measurement, which can be fitted in practically any eyepiece.

In the case of the conventional biological microscope, the top of the eyepiece is unscrewed and the graticule is placed on the field stop, the aperture about one-third to one-half way down inside the eyepiece which governs the field of view. The field stop is where the image of the specimen as magnified by the objective comes to rest, and it is the image at this point which is magnified by the eye lens to give the final image you see through the microscope. Anything placed at the field stop will also always be in focus. However, note that there is a vast difference between the eyepiece micrometer, the graticule - the glass slip-in disk just mentioned, and a micrometer eyepiece.

Using with a stage micrometer.
When used for the measurement of a specimen, graticules normally need to be used in conjunction with a stage micrometer, this being a standard size 3" x 1" glass slide engraved with a scale which is placed on the microscope stage. Graticules and stage micrometers are generally available from microscope dealers, but they are fairly expensive; graticules cost up to 10 and stage micrometer slides about 20 to 25. Graticules can usually be purchased in 19 and 21 mm diameter sizes with cross-lines, scales or square grid to fit conventional eyepieces on biological microscopes, and 25 mm diameter for stereo microscopes.

Incidentally, for our beginner members, note the distinction between micrometer, the scale etched on a glass disk or on a slide for measuring purposes, and a micrometer. The latter is an actual unit of measurement. Owners of the Russian MBS-9 and MBS-10 stereo microscopes are fortunate in that supplied as standard equipment with these microscopes is a special x8 eyepiece with dioptric focusing eye lens and with both scale and grid eyepiece graticules supplied for measurement purposes.

These two particular Russian stereo microscopes, when used with their eyepiece graticule, do not need a stage micrometer slide since the manual gives conversion tables to show the value of the specimen which corresponds to one division of the eyepiece scale or grid at different objective magnifications. In the MBS stereo microscope eyepieces, the graticule is inserted in the bottom of the eyepiece; this means that because this position is not a normal field stop, the graticule will not be in focus when looking through the eyepiece. This is why this is a focusing dioptric eyepiece; with it you first use the dioptric adjustment to bring the graticule into focus with the eyepiece, then you use the normal focusing control of the microscope to bring the specimen into focus in the normal way. The two graticules supplied with the MBS-9 are 23mm diameter and those with the MBS-10 are 25 mm diameter.

Using a Squared Graticule for drawing.
Most of us are no doubt familiar with the method of copying a map, a picture or a drawing, by either ruling the drawing in say " squares or, to save marking the drawing, to cover it with a sheet of tracing paper ruled with the squares. The paper on which the drawing is to be copied is ruled with say " or larger squares and the method is to copy the drawing one square at a time to a corresponding square on the larger ruled paper. Correct proportions are maintained, resulting in a more reasonably accurate larger size drawing than could be obtained freehand.

The same principle can be used with the squared graticule in the eyepiece which, when looking through the microscope, will be seen superimposed over the image, just like ruling a drawing with small squares, and the drawing hand copied one square at a time to the sheet of paper.

The Micrometer Eyepiece.
I have mentioned the Eyepiece Micrometer, which is not the same as a Micrometer Eyepiece, the latter commonly known as a Filar/Curtain Micrometer Eyepiece, which fits in place of the standard eyepiece. It has its own fixed eyepiece, often x10. At the side of the instrument is a calibrated wheel with scale and marker. Looking through the fixed eyepiece shows cross hairs, one horizontal, one vertical. Turning the calibrated wheel moves a further vertical hair line across the field of view like the closing of a curtain, which is how this type of micrometer eyepiece gets its name.

by Roy Winsby.

Note: This article was written by Roy Winsby and was published in the newsletter of The Manchester Microscopical Society - Newsletter 32. It has been reproduced here through the kind permission of Roy Winsby.


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