An example of brass of the composition 50 per cent, copper and 50 per cent, zinc was shown, each of the grains being composed of β, while the separated material was γ. The γ had separated round the grain boundaries and also inside the crystal grains. The specimen was etched with acid ferric chloride. When etched with mercurous nitrate, a totally different effect was obtained, as the y now etched black.

Mr. Mohn then explained the method of heat tinting, by means of which the various constituents of the specimen acquired oxidation films, the colours of which were dependent upon the temperature of formation. Certain parts of a specimen were apt to oxidise more rapidly than others. In the case of a heat-tinted steel containing phosphorus it was found that while the purer portions were tinted yellow the richer phosphorus portions were tinted red or purple owing to deeper oxidation. Every specimen passed through a colour-scale oxidation, but certain portions faster than others, forming an oxidation tint in advance of the others.

Stead's reagent, an acid solution of magnesium and cupric chlorides, was also used for testing for phosphorus in steel. If the specimen was etched with Stead's reagent, it would be found that on the phosphorus free portions, copper was deposited readily from the etching solution, but by no means so readily on the phosphorus rich portions of the specimen.

The objects being opaque it was necessary to have illumination by vertically reflected light. Oblique illumination was only of value when dealing with fractures and micro-structures on the original surface. When low-power objectives were being used there was a considerable working distance between the objective and the specimen, and it was, therefore, possible to employ some form of parabolic mirror arrangement such as the Sorby type, or a simple glass plate placed at an anile of about 45° between the specimen and the objective. Very good results were obtained by this method in low-power photomicrography. In the case of high power objectives the working distance is so small that it is impossible to put any form of illuminator between the specimen and the objective, necessitating the use of the illuminator inside the tube of the microscope. The two forms used were known as the " plane glass " illuminator and the " prism " illuminator. The former device is most suitable for high-power work, while the latter device operates excellently in low-power work up to 200 or 300 magnifications. Very good contrast is obtained with the prism illuminator because of the total reflection of the beam of light, but the resolution is poor, due to the prism cutting down the effective aperture of the objective. Also the specimen is not illuminated perfectly vertically. There is a considerable loss of light with the plane glass illuminator, thus reducing the contrast approximately 75 per cent, of the light passing right through the glass, but the resolution is much superior as it enables the full aperture of the objective to be used.

Mr. Mohn then described in detail the method of using the " Leitz " large metallurgical photomicrographic apparatus which was the type used for making the illustrations of the specimens exhibited during the course of the lecture.

A number of slides of a specimen of white cast iron showing laminated pearlite were shown. All the illustrations had been taken of the same portion of the specimen.

These photographs were taken over a range of magnification from 180 to 1,900 diameters, using a 14 mm. Achromatic objective, an 8 mm. Apochromat, and a 4 mm. Apochromat, together with Projection eyepieces I., II., and III.

Each combination of objective and eyepiece was used with " Prism " and " Plane glass " illumination, and each combination of objective, eyepiece and illuminator with blue and yellow light.

The photographs showed in particular, the superior images obtained when most of the magnification was done by the objective, and how the resolution was dependent on the numerical aperture of the objective, the eyepiece having no effect in this direction.

The superior contrast of the prism illuminator and the superior resolution obtained with the plane glass was also demonstrated.

The necessity of using yellow light with achromatic objectives was shown, but the attempt to obtain better resolution by using blue light of shorter wave length in apochromatic objectives was nullified by the appearance of violet fringes, diminution in contrast, and difficulty in focussing. In focussing, it was not a reasonably practical method to focus with yellow light first and then alter the focus to conform with blue light. It was always advisable to focus with the light to be used.

The photographs had been taken with Imperial Non-Filter Plates, and Mr. Mohn stated that, in practice, instead of making exposures upon any theoretical basis such as the square of the diameter of the eyepiece (n.a.), he preferred to use test strips in order to obtain what he considered to be the best exposure.

The lantern slides exhibited had all been made with Wellington Slow-Contact Plates.

With an increase of numerical aperture there was a decrease in the depth of focus; nevertheless for very high-power work it was better to use an oil-immersion objective. It was a question of getting the best possible result from the objective and leaving as little as possible for the ocular to do.

It was quite possible in the polishing of a specimen there would be a " flow " or " melt " or the metal owing to the friction, particularly in the case of soft metals, such as lead alloys, which were exceedingly difficult to polish. The softer material got dragged over, perhaps, a harder constituent. In the polishing of a material which was not perfectly stable there might be a kind of annealing effect on the surface, resulting in an alteration of grain size.