Moreover, the substage fittings may be provided with condensers differing in their apertures and their optical correction. Condensers of high aperture afford a means of bringing out the full resolving powers of an objective and possess the further advantage that they are available for the purpose of producing dark-ground illumination.

When used dry, i. e. without the interposition of an immersion fluid, the condenser cannot have a greater aperture than 1.0 N. A. since rays inclined beyond a certain limit are totally reflected. For ordinary observation in transmitted light the dry condenser amply suffices and has the advantage of being simple and easy to manipulate. In more exacting cases, where it becomes necessary to employ the full aperture of the condenser, the gap between the latter and the object slide should be filled up with a layer of oil.

Under these conditions it is most desirable to have the object embedded in a highly refracting medium to obviate total reflection at the upper surface of the object slide. Spherical and chromatic correction have an important bearing upon results in photo-micrography and observation by the method of dark-ground illumination, where much depends upon the absence of colour and the brightness of the field.

No. fe d. s. Codeword

80. Large substage, Type a*). This apparatus is made up of the following parts:

Plane and Concave Mirror of 50 mm diameter, above which are mounted a Swing-out and Rotating Iris-Diaphragm in a carrier fitted with a Rack and Pinion for oblique illumination and a Cylinder Iris Diaphragm, together with a Swing-out Two-lens Condenser having a numerical aperture of 1.20 (No. 99 j. The condenser and iris-diaphragm are made to slip into a sprung sleeve.

A small recess is provided in the mount below the iris-diaphragm for the accommodation of ground glass and coloured glass discs, which are supplied with the substage for use with artificial light.

The substage is fitted with rack and pinion by means of which it may be raised and lowered in an axial direction so as to focus the illuminating pencil in the most advantageous position relatively to the plane of the object . 3.15. 0. Illuminant

*) Stands A and B are invariably provided with a substage of the A pattern fitted with a three-lens condenser No. 100. If desired, Stands C, D, la and lb may likewise be equipped with a substage of this type.

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