Growing Crystals by Instantaneous Photomicrography . 491 



that its centre of gravity coincided with its point of support^ 

 and the friction of its bearings was so adjusted that it w r ould 

 move easily, and yet remain stationary during the return 

 stroke. The distance through which the observed object was 

 moved was easily varied by altering the relative lengths of 

 the lever-arms ; distances varying from one tenth to one 

 fiftieth of a millimetre were usually used. The shutter was 

 so arranged that during the exposure the segment and slide 

 were at rest, the shift in position being effected during the 

 four fifths of the revolution through which the shutter was 

 closed. The accompanying diagram will make the arrange- 

 ment clearer (see p. 492). 



As a source of light any ordinary combination of incandes- 

 cent electric lights proved to be inadequate. A good Auer 

 von Welsbach light with a powerful reflector was more satis- 

 factory, but the best results were obtained with the help of 

 sunlight directed by a suitably arranged mirror and condensed 

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

 difficulty of this arrangement was the great heat caused by 

 the converging rays ; a difficulty which was obviated partially 

 by an absorbent screen in later experiments*. 



The first photographs were taken by reflected light, the 

 drop of solution being placed upon a ruby-coloured slide. 

 As soon as the crystallization had begun upon one edge of 

 this drop, the very sensitive plate was uncovered and the 

 shutter and segment were set in motion. The exposure was 

 stopped after fifteen or twenty revolutions, so as to avoid 

 confusing 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 |. A ^sicol 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 crys- 

 talline structure was allowed to emerge. It is true that this 

 method could not in all probability decide the chief point at 

 issue ; for the prenatal globular condition of crystals would 

 probably have no effect on polarized light. Definite optical 

 structure is of course necessary to produce the required 

 deflexion of the plane of polarization, and such definite 



* Hutcliins has shown that pure water is as good as a solution of alum 

 for this purpose (Am. Journ. Sci. cxliii. p. 52Q, 1892). 



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



