RICHARDS AND ARCHIBALD. — GROWING CRYSTALS. 345 



As a source of li^ht any ordinary combination of incandescent electric 

 lights proved to be inadequate. ■ A good Auer von Welsbach light with 

 a powerful reflector was more satisfactory, but the best results were ob- 

 tained with the help of sunlight directed by a suitably arranged mirror 

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

 difiicultv of this arrangement was the great heat caused by the converging 

 rays, a difficulty which was obviated partially by an absorbent screen in 

 some later experiments.* 



The first photographs were taken by reflected light, the drop of solu- 

 tion being placed upon a ruby-colored slide. As soon as the crystalliza- 

 tion had begun upon ohe edge of this drop, the very sensitive plate was 

 uncovered and the shutter and segment were set in motion. The expos- 

 ure was stopped after fifteen or twenty revolutions, so as to avoid confus- 

 ing superpositions. Even with the strongest light the images were very 

 faint and unsatisfactory; it is not worth the space to reproduce them 

 here. 



Another mode of obtaining light images on a dark ground, applicable 

 to all except the isometric system of crystals, is the use of polarized light. t 

 A Nicol prism was placed in the barrel of the microscope, and another 

 just below the stage. The main body of the light was thus intercepted 

 by the crossed prisms, and only that which had been deflected by the 

 crystalline structure was allowed to emerge. It is true that this method 

 could not in all probability decide the chief point at issue ; for the pre- 

 natal globular condition of crystals would probably have no effect on 

 polarized light. Definite optical structure is of course necessary to pro- 

 duce the required deflection of the plane of polarization, and such definite 

 structure miglit not be possessed by the globules. Nevertheless, 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 impressions, each exposed ^ 

 second, could be obtained in a second. For this low power the eyepiece 

 was removed from the microscope and an objective with long focal dis- 

 tance alone was used to give the image. The degree of enlargement was 

 obtained by actually measuring the image of a micrometer scale divided 

 into y^^ millimeters. 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 crystalliza- 



* Hutching has shown that pure water is as good as a solution of alum for tins 

 purpose (Am. J. Sci., 143, 526 [1892]). 



t This suggestion was kindly made by Professor E. C. Pickering. 



