OIL LAMPS.
The illuminants already considered are excellent in every respect, but they are somewhat expensive. For ordinary photomicrographic work with magnifica tions up to about 400 diameters, the homelier paraffin lamp or incandescent gaslight will be found, when properly ap plied, to be perfectly satisfactory. They do not give nearly the same intensity as those already considered, and therefore the time of exposure is more prolonged ; this, however, is only a slight drawback in practice. With very high powers they are, perhaps, inadmissible, because the light is so faint on the screen that it is impossible to focus with any accuracy. The lamp should be one with a broad wick, and this should be kept carefully trimmed, the best result being obtained when the edge of the flame is set in the Dptical axis. Between the flame and the microscope a bull's-eye condenser is placed, and the rays are brought to a focus by placing a white card on the stage of the microscope and moving the condenser ap until a fairly clear modified image of the flame is seen upon the under side of the card. The lamp and condenser are now
in the best position for obtaining good illumination ; and, in order to save time in subsequent work, it is advisable to make pencil marks on the baseboard where they stand, so that they may be always placed in the same positions. A better plan, however, is to fix a few small pieces of wood by means of screws to the baseboard where the lamp and condenser are placed ; these pieces of wood then act as stops or indicators. Fig. G79 shows a simple photomierographic apparatus made by R. and J. Beck, in which an oil lamp and condenser are used. Another what similar but more elaborate arrange ment, by Ross, is shown by Fig. 680. The superstructure is fixed on a solid mahog any baseboard and all parts are exactly central to each other.
denser, and a substage condenser on the microscope, it requires about 30 seconds' exposure for an object magnified 200 diameters.