ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 365 



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

 Thus, when the shutter made two revolutions in a second, the exposure 

 was one-tenth of a second. A Henrici hot-air motor, combined with 

 speed-reducing double pulleys, enabled the experimenter to use any 

 rate of revolution desired ; the power was communicated by the pulley 

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

 make it absolutely so. Tbe sensitive plate or gelatin film was held 

 above in a suitable holder A, which was put in the place of the ground- 

 glass plate used for focussing just before each series of exposures. In 

 a first series of experiments it was arranged so that the image should 

 be bright on a dark ground, and for this purpose it was found more 

 convenient to move the crystallising solution than to move the photo- 

 graphic plate. For effecting this the slide G 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 E. The segment was moved gradually 

 by the oscillating motion of a connecting-rod D, 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 slightly exceeded the distance between the saw-teeth. 

 The segment was suspended in such a way that its centre of gravity 

 coincided with its point of support, and the friction of its bearings was 

 so adjusted that it would move easily, and yet remain stationary during 

 the return stroke. I is a weight for balancing the segment. The dis- 

 tance through which the observed object was moved was easily varied 

 by altering the relative lengths of the lever arms : holes H bored at 

 distances varying from one-tenth to one-fiftieth mm. were generally 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. Fig. 80 

 represents the apparatus an instant before an exposure begins. J is a 

 horizontal projection of the revolving shutter in detail. The best light- 

 source was found to be sunlight directed by a suitably arranged mirror 

 and condensed by reflectors and lenses. The chief, though not serious, 

 difficulty of this arrangement was the great heat caused by the con- 

 verging rays, a difficulty which was obviated partially by an absorbent 

 screen. The first trials were taken by reflected light, but were less 

 satisfactory than those obtained by polarised light. The images were 

 now much more clearly defined, but the magnification (30 diameters) 

 was too low to warrant conclusions about the genesis of crystals. 

 Among other substances, sodic nitrate, boric chloride, cupric sulphate, 

 and ferrous ammonic sulphate were found to give satisfactory images. 

 A photomicrograph shows the crystals of sodic nitrate obtained under 

 a higher magnification of 110 diameters with an exposure of *12 second. 

 Light-ground illumination was now tried, as it was considered that an 

 initial globular condition, if it ever existed, would probably not be 

 visible through the nicols. The slide and crystallising solution were 

 allowed to remain stationary, and a 2 '5 in. Eastman cartridge gelatin 

 film was moved as in the common film-carriers. At first a power of 

 100 diameters was employed, and very satisfactory pictures of the growth 

 of crystals of potassic iodide were obtained. They do not, however, 

 reveal anything new. Higher powers of magnification were used and 



Jan el8th, 1902 2 B 



