The production of high-power photo-micrographs is hardly a subject that can be described in these pages; the apparatus is costly for, even if one dispenses with a specially designed micro-photographic camera, it is necessary to have a good microscope and bellows camera, for really advanced work. Messrs Swift & Sons supply an excellent fitting, with which quite satisfactory photo-micrographs may be taken. It consists of a light metal cone, the more pointed end fits over the upper portion of the microscope tube; the other end of the cone is provided with ground and plain glass focussing screens and a dark slide. When using this apparatus, it is first necessary to find and focus our object in the ordinary way, before attaching the photographic apparatus. Having secured our object exactly as we wish it to be depicted and well in the centre of the illuminated circle, we remove the eyepiece and slip the metal camera over the top of the microscope tube. If now we place the ground glass screen in position, we can see an image of our object upon it. Great attention must be paid to the lighting, it is necessary that the illumination be perfectly even, otherwise our negative will be over-exposed in some parts and under-exposed in others. When everything is in order the plain glass screen is substituted for the one of ground glass. On this we shall probably not see any image with the naked eye, but with the help of a magnifying glass we can see a much finer image than was possible on the ground glass screen. The final focussing is done at this stage and, having secured as sharp an image as possible, the focussing screen is removed and the exposure is made. Experience alone will teach the length of exposure, it depends upon the amount and nature of the light, upon the transparency of the object and also upon its colour. There is no more simple apparatus for taking moderately high-power photo-micrographs; for more advanced work the micro-telescope and the super-microscope (see Chapter XVII) used with a camera, will enable the user to obtain pictures of objects magnified as much as five thousand times. CHAPTER XXI HOW THE GLASS USED IN MICROSCOPES IS MADE Having described all the ordinary uses of the microscope and having also insisted that the objectives are the most important part of the instrument, there are probably many of our readers who may wish to know in what manner this wonderful glass differs from ordinary glass and how it is made.