Growing Crystals by Instantaneous Photomicrography . 493 



structure might not be possessed by the globules. Never- 

 theless, the idea seemed well worth a trial. 



The images were now much more clearly defined and striking, 

 and with a magnification of 30 diameters, ten sharp impres- 

 sions, each exposed ^ second, could be obtained in a second. 

 For this low power the eyepiece was removed from the micro- 

 scope, and an objective with long focal distance alone was used 

 to give the image. The degree of enlargement was obtained 

 by actually measuring the image of a micrometer-scale divided 

 into j-q millimetres. The rapidity of exposure was so great 

 that many plates were sacrificed, for it was difficult to find 

 the precise moment when nascent crystals were in the field 

 of view. In most cases the crystallization was already well 

 started when the exposure began, as in fig. 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 difficultv was the attaining of the exact actinic focus, 

 which differed slightly from the visual focus. It was found 

 that a definite fraction of a revolution in the fine adjust- 

 ment of the instrument, or a definite shift of the position 

 of the sensitive plate, could be relied upon to cover this 

 difference, when 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 VII. figs. 2,3,4,5.) 



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 first appear as points, 

 indicating a diameter of less than -..-i-- millimetre. The 

 regularity of growth of those already well started is worth 

 a passing mention. 



The next objective used gave a magnification of 110 dia- 

 meters. With this power the light was so much diminished 

 that exposures of less than fe second became too pale. Three 

 examples from among these negatives are given here. 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 



* Of course the use of an apochromatic lens would have saved us 

 from this minor difficulty ; but our method of working accomplished the 

 object with less expense. 



