STEREOSCOPIC PHOTOMICROGRAPHY 193 



taking two photographs from positions on either side of 

 the perpendicular on the centre of the object, and com- 

 bining prints or transparencies from the negatives in a 

 stereoscope. If a half -plate camera is used there is no 

 difficulty in getting the two negatives of an object 

 illuminated as above described. Two distinct dots 

 should be made on the ground glass 2-J" apart, and 

 equidistant from the centre. The object is focussed and 

 so arranged that the centre of the image coincides with 

 one of the dots, and a photograph is taken ; the object is 

 then moved until the centre of the image coincides with 

 the second dot and another photograph taken, the 

 exposure given being the same in both cases. Both 

 photographs can be taken on the same half-plate if one 

 half of it be covered by an opaque card previous to the 

 first exposure, after which the slide is taken to the dark- 

 room and the card placed on the exposed part of the 

 plate ; the second exposure is then made on the un- 

 exposed half. The two images will develop up to the 

 same density if correctly exposed. If it is preferred to 

 use quarter-plates, suitable carriers can be cut out of 

 black cardboard, or dark slides are obtainable constructed 

 to carry two quarter-plates or a special stereoscopic size, 

 and with shutters that permit of two exposures being 

 given. As a rule the condenser and lamp will require re- 

 adjustment before the second exposure is made, as the 

 movement of the object affects the equality of the 

 illumination. A mechanical stage is of great service for 

 stereoscopic work, as it obviates any difficulty in the 

 movement of the object from one position to the 

 other. 



The need for moving the object, with the readjustment 

 of the illumination thereby rendered necessary, can be 

 avoided either by moving the objective on a sliding or 

 centring nose-piece to the extent required to form the two 

 images on the ground glass, or by covering up first one 



