Colour Filters

In all Objectives other than those of the Apochromatic series, there is of necessity a residual colour in the corrections.

The working of a good Objective can be made equal to that of the Apochromatics by the use of a colour filter. If this filter has a definite range of wave-length only the one colour can pass, all others being absorbed, and the best result is in consequence obtained.

Coloured glasses and light filters have, however, another province of usefulness. They are invariably used for photo-micrography to produce contrast effects in coloured objects, but it does not seem to be understood that they are valuable for visual work.

There are certain colours, used for double-staining and other bacteriological work, which do not stand out with sufficient strength by themselves, and the resulting image depends on the relation of the outstanding colour in the Objective to the staining of the object.

For this reason we always supply a disc of yellow glass with our 1 /12th in. oil immersion objectives, to strengthen the contrast in bacteriological specimens.

General Hints

Medium and high power objectives should never be taken apart, because they are centred in the position in which they are screwed when sent out. If, by any chance, they should become dirty, they should be returned to the makers. Care should be taken to keep the front lens clean, the best material to wipe it with being an old silk handkerchief or a piece of linen thai has been washed many times.

It occasionally happens that a lens is returned to the makers with the remark that it has suddenly ceased to perform well, and on examination it is found that a very fine film of Canada balsam or other material used for mounting objects has covered the front lens. Being of a transparent nature it is not quickly detected. We suggest that this should be looked for with a pocket lens, if a sudden lack of definition arises.

It will be found best always to commence the examination of an object with a low power, subsequently using a higher one, as may be found necessary. A much better idea of the whole subject is obtained in this way.

Eyepieces. It has been previously stated under the heading of Objectives that the initial powers of an objective and an eyepiece multiplied together give the magnifying power that is being used.

High-Power Eyepieces

It frequently happens that eyepieces of the higher powers are ordered, and disappointment ensues with the results obtained. Increase of magnification is secured with them, but usually with diminution of definition and a darkened field. All regular observational work is done with eyepieces 1 to 4, and the deeper powers should only be used for occasional reference purposes.

The rule should be to use the eyepiece which is sufficient to exhibit the detail in the object, and no more.

The Field

Considerable misconception exists regarding the diameter of field that is seen. The extent o( the object that will be embraced and seen at one time is naturally greater with a low than with a high magnification, but we are referring now to the diameter of the circle or field that is seen when looking through the instrument. This is controlled entirely by the diaphragm in the eyepiece- if this has a large opening the diameter of the circle is increased, of small, it is diminished. The diaphragms are all made to give the circle or field that is most advantageous and convenient, and although a large field may be at times useful for low-power work when following the movements of living objects, etc., it is distinctly disadvantageous with objectives of medium and high magnification, because only one part of the object is seen sharply at one time -see note re Flathess of Field, page 201.

Scales And Verniers

The method of reading the vernier is this : First the Zero line of the vernier is used like an ordinary index to find the whole number of millimetres of the scale : in the figure the Zero line points between 12 and 13 of the main scale, hence the whole number is 12. To find the fractional part to be added to this, the other lines of the vernier must be examined in order to find that which most nearly falls into exact alignment with a line of the main scale. This having been done, the corresponding number of the figures engraved on the vernier : the first of these being 0, the last 10, one sees at a glance that if all the lines of the vernier were numbered, the one at which coincidence takes place in Fig. 126 would bear the number 7 ; hence the complete reading of the vernier in this figure is 12-7 mm.

A little practice will soon enable this to be done with facility.

Fig. 126.

Cleanliness

It is very important that a microscope should be kept in clean condition. Where it is constantly exposed in the laboratory a ready means of protection is to be found in a bell glass cover, which can be put over it quickly when not in actual use. The instrument should be carefully wiped before use on every occasion.

Never oil the bearings of the microscope unless quite sure that the oil is perfectly pure and free from acid. Watchmakers' oil is the best.

Never use the immersion oil for lubricating the fittings of a microscope. It is really of a resinous nature and works havoc with the fittings.

Above all things, it is desirable that those who use the microscope should be cleanly. Lacquer is soluble in spirits of wine. Fingers which have been dipped in spirituous media should not be brought in contact with the microscope while in a moist condition ; otherwise its appearance will be spoilt. Care should be taken not to touch it with hands covered with media and reagents.