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330 THE ADVANCE OF PHOTOGRAPHY
is wise to work with a slow developer so as to be better able to watch the development and to secure the required amount of contrast. If prints are made, choose some paper with a good glossy surface so as to bring up the details of the image to the best advantage.
Cameras for Microphotography in Metallurgical Work. — There is one important branch of micro-photography, viz. the metallurgical, in which one is concerned very largely with opaque specimens. Fig. 141 shows the usual laboratory arrangement for this kind of work. A is the arc lamp, B and D are condensers, C is the cooling trough, E diaphragm, and F 8 the microscope. Although the beam of light is passed through a trough to lower its heating power, it is still not passed directly through the miscroscope, but it is reflected so that it ultimately passes forward in a direction parallel to its original course ; in other words, the camera and microscope are parallel to, but not in the same straight line as the beam of light in the right-hand part of the apparatus. By means of such instruments it is possible to obtain very fine photographs of the surface appearance of metals, and also their intimate structure after undergoing any desired treatment. This is clearly illustrated in the next two figures.
The first of these shows a specimen of carbon steel containing 0*89 per cent, carbon, which has been heated to 1000° C, and then allowed to cool. The magnification in this case is 960 diameters. The second shows the surface structure of a silver cupellation bead, and the magnification is 120 diameters.
Quite recently a very neat form of apparatus suitable for a lecture-room bench has been devised by Messrs Leitz. It has the great advantage over the one just described that it does not take up more than one-half the space.
The stand consists of an upright which carries the tube and appliances for illuminating the object. Above this the stage can be seen. This may be raised or lowered