If the mirror of the microscope be of good size, it will only be necessary to make an arm on which to support the removed mirror outside some southerly exposed window, since it is desirable to have a greater distance between the mirror and the stage than would be possible were the mirror attached in its usual place. Where the microscope mirror is too small to be satisfactorily used, a rectangular wood-framed looking-glass is readily mounted, with the aid of a few strips of wood, so as to turn about both axes.
The rays from the plane side of the mirror should pass through a condensing lens (of 8-10-inch focus, if possible), so placed that they are brought to a focus before reaching the plane of the object. The exact position of the condensing lens is a matter of experience; usually, however, the most favourable illumination is obtained at that point where the field is brilliantly and uniformly illuminated, just before the rays form the image source of light; the nearer the focus the less disturbance from diffraction rings. Ordinary objectives will require the employment of monochromatic light—produced either by a deep blue solution of ammonia-sulphate of copper, or by the green glass screen—since the optical and actinic foci do not usually perfectly coincide. Powers up to the ¾-inch will require no further condenser; with the ¼ or 1⁄6-inch objectives, the low power (1 or ¾-inch) serves with advantage as an achromatic condenser, when attached to the sub-stage. The Abbe condenser, although so important for fine microscopical investigation, is not adapted to photography unless a very wide cone of light is desired, which, for the majority of preparations, is some advantage; a low-power objective, used as a condenser, is found to be more satisfactory than the Abbe with a small diaphragm.[30]
The greatest delicacy in manipulation is necessary, as in working with a 1⁄12-inch objective a turn too much of the fine adjustment will cause the image to vanish. With fine preparations of bacteria it is not easy to trace the image, and hence the advantage of commencing with a well-marked object, as that of the fly’s tongue. The development and fixation of the image must be proceeded with as in the ordinary photographic process. In the text-books of photography full accounts of failures will be found, their causes and prevention. Numerous papers and suggestions for micro-photographic work will also be found scattered throughout the “Journal of the Royal Microscopical Society.”
The Projection Eye-piece has become an essential part of micro-photography, and it is so arranged that it may be employed with advantage with objectives of either the apochromatic or ordinary series for photographic purposes, projecting an exquisitely sharp image of the object on the plate. A diaphragm between the lenses limits the field, and a sharp image of it should appear on the screen when the eye-piece is adjusted. The adjustment may be effected by revolving the eye-piece cap in a spiral slot, so that the eye or top lens is either brought closer or removed farther away from the diaphragm, as may be required, and divisions and a reader are usually provided for registering positions. Such eye-pieces are made to fit any size microscope body.
| Initial magnifying powers:— | ||
| English length of tube | 10-in. | 3 and 6. |
| Continental "" | 6-in. | 2 and 4. |
Fig. 173.—Baker’s Pringle Vertical Micro-photographic Apparatus.
The microscope and camera ([Fig. 173]) are here seen to be part of the same instrument. The bellows of the camera have an extension varying from 6 in. to 30 in. The board on which the microscope and limelight jet are fixed is made to turn out of the line of the camera to facilitate adjusting the instrument and radiant, either limelight, electric light or paraffin lamp; when this is done the board carrying the same is turned back to a stop which brings the microscope into a central position with the focussing screen. An adjustment is supplied at the side of the camera, geared to the slow movement, for finely focussing the object upon the screen. A light-excluding connection is fitted to the front of the camera and microscope; immediately behind this, in the bellows, is an exposing shutter which is manipulated by means of a small milled head. Two focussing screens are usually supplied, one grey, and one patent plate, together with a double dark slide.
Mr. Andrew Pringle’s vertical micro-photographic apparatus is an excellent form; it consists of a heavy base and brass support, carrying a quarter-plate camera, grey and plain glass focussing screen, double dark back, camera extending to 24 inches, and turning aside as shown in Fig 173. It is light-tight in all its connections.
To secure uniform results in micro-photography, only thin preparations, which lie as nearly as possible in one plane, can be relied upon for good and perfect negatives.