GENERAL PBELIMINAEY PKEPARATIONS 189 



to the same source, as in even the best microscopes it is not 

 unusual to find that the optical axis of the instrument is not 

 truly horizontal. These errors are most easily detected by 

 watching the projected image in the manner described, since 

 this forms a far more exact test than any method of direct 

 measurement. 



Methods of Exposing. In many ready-made cameras an 

 exposing-shutter is fixed at the front, which is actuated by 

 mechanical or other means. The great disadvantage of all 

 such arrangements is that there is the danger of some vibration 

 being imparted to the apparatus ; and as such vibration 

 obviously occurs at the moment when an exposure is being 

 started, the result may be most harmful. 



It will be found far more satisfactory to have some very 

 simple arrangement for effecting the exposure. The simplest 

 of all is merely to interpose between the microscope and the 

 nearest auxiliary condensing-lens a piece of blackened cardboard. 

 This may be supported in position as near to the microscope as 

 possible, but yet not touching any part of the instrument, and 

 may be removed at the commencement of the exposure and 

 replaced in its position at the end. It will not be found, at 

 least when working in a room which is even partially darkened, 

 that a sufficient amount of reflected light enters the camera to 

 have any effect on the photographic plate. In cases where the 

 work is carried on in a well-lighted room, some arrangement 

 inside the camera may become necessary. The simplest 

 way is then to have a thin metal plate, which should be as 

 light as possible, carried inside the camera front on a hinge 

 or thin spindle, with freedom to rotate. The plate should be 

 rigidly connected to the spindle, and the latter may project 

 through the front of the camera on either side, with a milled 

 head outside the camera front which may be actuated at either 

 or both ends. On rotating the milled head the plate should 

 be so arranged that it either completely closes the aperture 

 in the camera front, or moves downwards and lies in the bottom 

 of the camera, leaving therefore a clear aperture where the 

 microscope and camera join. 



Alternatively any of the mechanical photographic shutters 

 may be used, and may be fixed in the front of the camera. But 

 unless one of the modern between-lens shutters of good 



