It cannot be too strongly emphasised that if an Objective is to give its best result it must be used with the proper thickness of cover glass on the object, and with the precise tube length for which it has been adjusted.

The expert eye can detect an error in tube length of a few millimetres by the lessened defining power that results.

Flatness of field is so often asked for that we think it desirable to mention that this can never be obtained in other than low power Objectives, and this fact applies to Objectives by all makers.

'I heoretically 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 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.

With low powers-1in., 1/2in., 2/3in., etc., and those of less magnification, flatness of field can be secured over nearly the entire surface, but under no circumstances can flatness of field be produced in a lens of large aperture and fine quality of high power-it is an optical impossibility.

The Objectives that are to be chosen will depend on the class of work that is to be undertaken.

The ability of a lens to define fine structure is associated with its numerical aperture, the greater the numerical aperture the larger the number of lines per inch that can be divided. If the unknown is to be investigated, and the limits of resolving power are to be available, Objectives with the largest numerical aperture that can be obtained are essential.

The greater part of the work that is done, however, does not call for the use of such lenses. Generally the aim is to see with the utmost clearness, structure which is known and probably described, and for such, lenses of sufficient though comparatively low aperture are requisite.

In choosing Objectives it must be borne in mind that the great numerical aperture necessitates reduction of working distance and the flatness of field. As a general recommendation we should say that our Parachromatic series will be found to cover the wants of all those who are engaged in work of a general educational and recreational character, but for those who desire the best obtainable, or are doing research work, lenses of the Holoscopic or Apochromatic series should be chosen.

The Biological student invariably takes the fin. (16 mm.) and l/6in. (4 mm.), and when bacteriology and ha?matology are studied the 1/12in. oil is immersion added to these.

The amateur will find himself advantageously placed with similar objectives, but he will frequently need objectives of lower magnification such as the 2in. and even on occasion the 3in.

Objectives of specific focus are always the best, but to obviate the expense of two or three objectives, and to give a range of magnifications, variable power objectives can be obtained, the change in power being effected by rotating a milled collar; these will be found exceedingly useful for low power work.

In Objectives constructed entirely with optical glass, even in those of the finest quality, there remains one outstanding defect which is inherent in the material used, namely, the so-called secondary Spectrum. This arises from the fact that the distribution of colour in the Spectrum produced by the various optical glasses varies with their density, the red end being relatively too long in glasses of low dispersion, and the blue end too long in glasses of high dispersion, the result being that when such glasses are combined to obtain the best achromatism for the brightest part of the visual Spectrum, both the ends of the Spectrum, namely, the red and the violet, are refracted too little or in other words, come to a focus at a greater distance than the middle of the Spectrum, the chief inconvenience arising from the presence of this secondary Spectrum is that the Objectives cannot be used for photographing in white light on ordinary plates, but will only give satisfactory results if Isochromatic plates and effective colour screens are used. Occasionally the secondary Spectrum also interferes with delicate visual observations because the faint halo of purple light due to it falsifies the true colour of small detail.

In Apochromatic microscope objectives the removal of the secondary Spectrum depends upon the use of fluorite lenses, and the difficulty of procuring optically perfect fluorite has restricted output in past years. Special attention is called to the 4 mm. Objective of -85 N.A. as a lens which can be confidently recommended for general use as a high-power dry lens. It is absolutely impossible to obtain perfect correction of aberrations in the marginal zone of dry objectives of .95 N.A. It is for this reason that this new lens has been limited to an aperture of -85, and it will be found that on suitable objects this objective will bear practically a full Aplanatic cone of light. All those who know the difficulty of using a correction collar on objects of unknown structure under a cover glass of unknown thickness will also agree that the absence of the correction collar is at any rate not a drawback, as it guarantees perfect centring in the new lens, and excludes any fear of this perfection being lost through lost motion in the correction arrangement.

Fig. 127 illustrates an objective of 2 m/m focal length with a numerical aperture of 1.37 Oil Immersion of Messrs. Watson & Son's Apochromatic Series.

A Make-Shift Microscope

Take a small piece of glass, free from bubbles, not too thick, place on it a drop of glycerine, or balsam, or even golden syrup or water. View the object through the drop on the glass.

Fig. 127.