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lens. The centering nosepiece of this instrument is designed so as to be very steady. Below the stage is an achromatic convergent system, N.A. 1'0, mounted in an iris diaphragm provided with a rotating swing-out cell for carrying gypsum plates, oblique light stops, &c. The convergent system and diaphragm have centering screws. The hemispherical lens of the convergent system can be slipped out of the optic axis when light of less convergence is required. The polariser, which is a large Nicol prism, is mounted independently on an eccentric arm. It is divided on the lower edge of its rotating mount. The whole of the substage apparatus can be swung out of the optic axis, and is focussed by means of a quick-acting lateral screw.

Small Stage Goniometer. (Fig. 5.)

Fig. 5.

This instrument is clamped on the stage of a mineralogical miscroscope for determining the separation of the optic axes of bi-axial crystals, &c. It consists, essentially, of a pair of forceps or a needle point attached to a vernier, reading to 5', which moves round a divided semi-circle. The small mineral section, or cleaving, is held either in the forceps or with a touch of wax on the needle point. The forceps and point are interchangeable. The instrument is so constructed that the readings may be made with the mineral either dry or immersed.

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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 99,40 %.

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

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