494 Prof. Richards and Mr. Archibald : A Study of 



rapid growth o£ small particles has already been noticed by 

 Ostwald*; it is treated more fully in the following pages. 

 The crystals of sodic nitrate grew faster than those of baric 

 chloride or cupric sulphate, and the two latter substances 

 evidently appeared at first in very thin plates. It is interest- 

 ing to note that the thickening of these plates caused a 

 corresponding change in the quality of the emerging light, 

 and hence the crystal-images show a rhythm of dark and 

 light. (Plate VII. figs. 6 and 7, Plate VIII. fig. 9.) 



At this point the whole method of procedure was changed 

 on account of the probability that a globular condition, if it 

 existed at all, would not be visible through the crossed Nicols. 

 The apparatus was now arranged for the exposure o£ suc- 

 cessive portions of a film to unpolarized sunlight emanating 

 from a bright field, upon which the crystals appeared as dark 

 spots. The slide and crystallizing solution were allowed to 

 remain stationary, and the gelatine film was moved, as it is 

 in the common film-cameras. The 2J-inch Eastman cartridge 

 film was found to answer the purpose. At first the turning 

 was effected by an automatic electromagnetic arrangement 

 which received its current from a make-and-break contact 

 attached to the shutter. Since a current of ten amperes was 

 needed to secure a sufficiently forcible and speedy action, the 

 operation of this device was somewhat troublesome, and when 

 the exposures were not much more frequent than one a 

 second the film was rolled by hand. A suitable signal 

 attached to the shutter axle, which was still turned by the 

 Henrici motor, gave the necessary indication of the proper 

 moment for renewing the sensitive surface. With this appa- 

 ratus it was of course possible to obtain photographs o£ 

 isometric crystals, which could not be examined with the 

 preceding arrangement. 



At first a power of 100 diameters was employed, and very 

 satisfactory pictures of the growth of crystals of potassic 

 iodide were obtained. One of these negatives is reproduced 

 here as an example. (Plate VIII. fig. 9.) They showed nothing 

 new, however ; hence a much higher power of 580 diameters 

 was applied by combining a " 2-inch " eyepiece with an 

 " J -inch " objective. With this contrivance the light was o£ 

 course far less intense, and the definition less sharp. Even 

 with the brightest sunlight, concentrated by mirrors and an 

 Abbe condenser, the exposure could not profitably be made 

 less than J- second. These plates have been enlarged by usual 

 processes to over seven times their original size, so that a 

 total enlargement of over four thousand diameters has been 

 * Ostwald, Zeits.phys, Chem. xxii. p. 320. 



