RICHARDS AND ARCHIBALD. GROWING CRYSTALS. 343 



regards expenditure of sensitized film, and the more simple in execution ; 

 for when the field is dark, successive images can be obtained by a very 

 slight motion of either object or film, while, when the field is light, the 

 whole previously exposed surface must be replaced by a fresh surface 

 before each exposure. 



The apparatus consisted of a good compound microscope fitted above 

 with a vertical folding camera, which was supported by two massive steel 

 pillars on the heavy stand. It was, in short, the regular photomicro- 

 graphic outfit made by Bausch and Lomb. Between the microscope and 

 camera, in a suitable light-tight box, was placed a revolving shutter, which 

 allowed an exposure equal to one fifth of the time of its revolution. Thus, 

 when the shutter ma<le two revolutions in a second, the exposure was one 

 tenth of a second. A Henrici hot air motor, combined with speed-reduc- 

 ing double pulleys, enabled the experimenter to use any rate of revolution 

 desired. The rate was reasonably constant, but no attempt was made to 

 make it absolutely so. The sensitive plate or gelatine film was held above 

 in a suitable holder, which was put in the place of the ground-glass plate 

 used for focusing just before each series of exposures. 



In carrying out the first of the two methods it was found more conven- 

 ient to move the crystallizing solution than to move the photographic 

 plate. For this purpose the slide bearing the drop of liquid was attached 

 by a wire to a point just below the centre of a segment provided above 

 with saw-teeth. Tlie segment was moved gradually by the oscillating 

 motion of a connecting-rod, fastened by a crank to the revolving shutter 

 at one end and playing into the saw-teeth on the other. In order to 

 make the motion certain, the stroke of the connecting-rod slio-htly ex- 

 ceeded the distance between the saw-teeth. The segment was suspended 

 in such a way that its centre of gravity coincided with its point of sup- 

 port, and the friction of its bearings was so adjusted that it would move 

 easily, and yet remain stationary during the return stroke. The distance 

 through which the observed oltject was moved was easily varied by alter- 

 ing the relative lengths of the lever-arms ; distances varying from one 

 tenth to one fiftieth of a millimeter were usually used. The shutter was 

 so arranged that during the exposure the segment and slide were at rest, 

 the shift in position being effected during the four fifths of the revolution 

 through which the shutter was closed. The accompanying diagrana will 

 make the arrangement clearer. 



