Beck Object Glasses.

The manufacture of object glasses has advanced in recent years. By the invention of new formulae and by improved methods of testing we may claim to be producing object glasses which are somewhat in advance of anything hitherto made in this or any other country.

In ordinary object glasses, two series are made, achromatic and apochromatic. The achromatic series fill the requirements for most purposes, and are in more general use on account of the fact that they are made to a simpler formula and of less expensive materials and are consequently able to be sold at a lower price. The resolution is of a high order and is, in all lenses, as near to the theoretical resolving power as is attainable. The chief feature of the apochromatic series is that different glass is employed and other materials substituted and this combined with a different formula involving the use of a larger number of component lenses produces an object glass in which there is a more perfect correction for chromatic and zonal aberrations. In achromatic object glasses the correction is made for two colours of the spectrum but in apochromatic lenses the correction is made for three colours. For very fine structure undoubtedly apochromatic lenses give superior results ; the perfection of the colour correction enables certain objects to be seen with greater crispness than is possible with achromatic lenses and higher power eyepieces can be used without breaking down the image. For photography the apochromatics have a decided advantage.

All object glasses, unless otherwise ordered, are corrected for a tube length of 160 m/m., and the drawtube should be extended to the 160 mark. The Sloan objective changer measures 10 m/m., and if this is used the tube should be extended to the 150 m/m. mark. Similarly if a revolving nosepiece is used which measures 10 m/m., the drawtube should only be extended to the 150 m/m. mark.

Dry object glasses, unless otherwise ordered, are adjusted for use with a coverglass -006 inch (•15 m/m.) thick, and a small thickness of mounting fluid, and coverglass of approximately this thickness should be used for all high power work. The exact thickness of coverglass to be employed for a dry object should be -007 inch (T8 m/m.) with the tube length of 160 m/m. or -006 inch (•15 m/m.) with tube length of 180 m/m. Object glasses specially ordered for use with uncovered objects such as metallurgical specimens are supplied at the same prices, unless short mounted, when a small extra charge is made.

All achromatic object glasses, except the lowest powers, 32 m/m. to 60 m/m. focus, have the same optical lengths, so that when used on a Sloan objective changer or a revolving nosepiece, they are in focus without more than a small movement of the fine adjustment.

For dark ground illumination it is often the practice to place a removable stop in the back of the object glass to reduce its aperture to the requisite amount to exclude direct light. This method has the disadvantage that the stop, not being in the most effective position for its purpose, it is necessary to cut down the aperture to a greater extent than is actually required, thus sacrificing the resolving power of the object glass. The inch (2 m/m.) and f inch (3 m/m.) 1-2 N.A. apochromatic and achromatic object glasses are specially made for use with our high power focussing dark ground illuminator with m/m. slides, the full aperture being employed. For thicker slides up to 1£ m/m. thick a stop is provided to reduce the aperture to -95 N.A. The inch (3 m/m.) N.A. -95, and the inch (2m/m.) N.A. 1, achromatic object glasses are suitable for use with any type of high power dark ground illuminator with a slide of approximately 1 m/m. thick, their full aperture being employed. All these object glasses are also suitable for use with direct illumination.

Object glasses to be used with the Beck aplanatic ring illuminator are supplied without extra cost specially mounted to allow of the full illuminating angle being utilised but lower powers than 16 m/m. require a special adapter.

Monochromatic object glasses are also made, one specially for metallurgical purposes with the exceptionally large aperture of N.A. 1-6, also a series is made for use with ultra violet light. These will be found described on the following page. These object glasses have been found most satisfactory in use by those who employ them for their special purposes and very successful work has been carried out with them.

For hints on the use and selection of object glasses see “ The Microscope, a simple handbook,” and for further details of illumination and technique see “ The Microscope, an advanced handbook.”

72

---

Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 99,36 %.

La langue de reconnaissance de l'OCR est l'Anglais.

---