Low Powers

Certain objects are of so transparent a nature that their beauty is not disclosed when ordinary transmitted light is used. By placing a black patch stop beneath the back lens of the condenser, the object stands out clearly upon a black background in a charming manner.

This method of illumination is particularly applicable to Pond Life subjects, etc., when examined with low powers-say up to Jin.

Owing to the shortness of focus of the ordinary substage condensers, difficulty is often experienced in illuminating objects contained in troughs, etc., with a black background.

A spot lens, on account of its long focus, will therefore often be found more suitable.

For High Powers

The " Holos" Immersion Paraholoid produces dark ground illumination with high power objectives. Its particular usefulness lies in the searching for and examination of living Bacteria, especially Spirochce, for which a high power objective is necessary.

No objective having a numerical aperture in excess of 1.0 can be used for dark-ground illumination, and as the 1 /12th inch oil immersion lens is generally preferred, its numerical aperture is reduced by placing a diaphragm of funnel shape in the interior of the mount of the Objective. Recently the Watson 1 /7th inch oil immersion Objective with a numerical aperture of less than 1 0 has been found to meet all requirements with a saving in equipment and cost, for such a lens may take the place of both 1/6th inch and 1/12th inch oil immersion for Objectives. The " IIolos " Immersion Paraboloid is sometimes referred to as an " Ultra " Microscope. The special conditions necessary for working it may be summed up in absolute centrality of condenser with Objective for which a centring substage is essential ; proper thickness of object slip as indicated on the paraboloid : the paraboloid must have a film of oil between its top lens and the under side of the object slip, and the oil contact must be maintained.

Nelson's Gassegrain Condenser

This Gassegrain Dark Ground Illuminator has the advantage over all other dark ground illuminators that it can be used with the full aperture of any Objective, without the insertion of the funnel stop.

This means that the elliciency of an oil immersion lens of 1 -30 numerical aperture can be fully employed, while with other dark ground illuminators the maximum aperture that can be utilised is 1.0 N.A.

The method of use is generally the same as with a IIolos Immersion Paraboloid, the necessity for absolute cent ration being emphasised on account of the larger numerical aperture and greater sensitiveness to exact adjustment.

It is not always easy to maintain immersion contact between the large surface of the top lens and the under side of the slide, and some more viscous material is generally recommended than ordinary immersion oil. Golden syrup has been found to be an excellent substitute.

Revolving Nosepiece

With the majority of microscopes a nosepiece carrying 2, 3 or more objectives is provided, so that examination may be made rapidly with objectives of different power. When this is fixed in position on the microscope by means of its screw collar, it should be finally clamped with the projecting part exactly outwards in a line with the body towards the front of the stage. It must not lie on either side, otherwise, when the tube is racked upwards, it will foul the bearing slides.

A revolving nosepiece requires to be treated with great consideration. A moment's thought will reveal the accuracy that is demanded in it, and the least strain will tend to make it incorrect. A habit should be acquired of always rotating the nosepiece in one direction and not to and fro. This applies especially to the triple nosepiece. This will materially help to maintain accuracy in centring.

Objectives : Magnifications

The magnification can always be roughly ascertained by remembering that it is secured by the eyepiece in conjunction with the objective, it only being necessary to multiply their powers together.

It may further be remembered that 1/2in., 1/4in., l/6th in., etc., does not represent the working distance of objectives, but the equivalent focus, on which the magnifying power chiefly depends, the lin. objective mantrifying approximately 10, the Jin, 20, the 1 /6th in. 60, the 1 /12th in. 120 diameters, and so on. All the eyepieces are engraved with a figure denoting their magnifying powers. Supposing a No. 2 eyepiece with a power of 6 were used with a l/6th in. objective, the magnification yielded would be approximately 6 x 60 = 360 diameters. This is reckoning for a 10in. tube length : if a longer or shorter tube length is used, the proportion is greater or less. 10 is made the denominator, the tube length employed the numerator, and the ascertained figure referred to above is multiplied by this fraction : thus-with a No. 2 eyepiece and 1/6th in. objective at 8 inches, the magnification would be, roughly, 360x8/10 = 288 diameters.

Flatness Of Field

This is mentioned especially to point out that this feature cannot exist in any other than lower-power objectives, and the fact applies to objectives by all makers. Makers are so often asked if they can supply high-power objectives with a flat field that we have thought it well to call special attention to this matter.

Theoretically and practically all microscope lenses have a curved field, and with high-power objectives the greater the excellence of the lens, the more particularly does this become apparent. It is possible, by altering the focus, to view separately every zone of the field, but when a specimen is focussed in the centre of the field, this part only is absolutely sharp. Flatness of field can only be obtained by sacrificing the maximum sharpness at the focal point, and this produces inferior definition. The necessities of many classes of work-render desirable as great a degree of flatness of field as can be discreetly given, and the judicious combination of that effect with the best possible definition in the circumstances is provided.