34G PROCEEDINGS OF THE AMERICAN ACADEMY. 



tion was already well started wheu the exposure began, as in Figure 4; 

 but in some nothing but blank negatives were obtained. The best method 

 is so to arrange circumstances as to have the crystallization begin upon one 

 edge and spread slowly over the drop. Another difficulty was the attain- 

 ing of the exact actinic focus, which differed slightly from the visual 

 focus. It was found that a definite fraction of a revolution in the line 

 adjustment of the instrument could be relied upon to cover this difference, 

 wheu experiment had once found the right spot. 



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

 and ferrous amnionic sulphate were found to give satisfactory results. A 

 few photographs chosen as being typical examples of many negatives are 

 given here. (Plate I. Figs. 2, o, 4, 5.) 



All the images recorded on these plates are perfectly sharp and regular 

 when in focus ; but the magnifying power was too low to give important 

 evidence concerning the birth of the crystals. The crystals always lirst 

 appear as points, indicating a diameter of less than ^^^ millimeter. The 

 regularity of growth of those already well started is worth a passing 

 mention. 



The next objective used gave a magnification of 110 diameters. With 

 this power the light was so much diminished that exposures of less than 

 ^ second became too pale. Three examples from among these negatives 

 are given below. It will be noticed that in all cases the crystals have 

 their regular forms when they first appear upon the plate. Another 

 point worthy of attention is the fact that the growth in diameter at first 

 is more rapid than it seems to be subsequently. This rapid growth of 

 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 sub- 

 stances evidently appeared at first in very thin plates. It is interesting 

 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. I Plate I. Figs. 6 and 7, Plate II. 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 

 lie visible through the crossed Nieols. The apparatus was now arranged 

 for the exposure of successive portions of a film to unpolarized sunlight. 

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



Bpots. The slide and crystallizing solution w<re allowed to remain sta- 

 • Ostwald, Zeita phys Chein . 22, ! 



