490 Prof. Bichards and Mr. Archibald : A Study of 



difficulty is increased by the fact that most crystals are so- 

 transparent as to absorb but little light, and reflexion is 

 possible only in certain directions. Hence it is hard to- 

 obtain a distinct image even in a strong light. Moreover, 

 the machinery necessary for shifting the plates must be so 

 frictionless in construction } and so firmly fixed, as to impart 

 no vibration to the camera or the mobile subject of study. 



These difficulties were at least partially overcome by two 

 different arrangements; the first of which caused the successive 

 impression of a bright image in a dark field, and the second 

 registered dark images in a succession of bright fields 

 Obviously the former was the more economical as regards 

 expenditure of sensitized film, and the more simple in execu- 

 tion ; 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 sup- 

 ported by two massive steel pillars on the heavy stand. It 

 was, in short, the regular photomicrographic 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 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 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 focussing 

 just before each series of exposures. 



In carrying out the first of the two methods, it was foi nd 

 more convenient 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. The 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 slightly exceeded the distance between 

 the saw-teeth. The segment was suspended in such a way 



