394 Prof. Marshall Ward. Experiments on the [Dec. 1 5, 



summer soon showed that precautions of several kinds were necessary. 

 The direct exposure of an ordinary plate-culture to the full light of even 

 a September or October sun, especially in the afternoon, usually leads 

 at once to the running and liquefaction of the gelatine, and although 

 the exposed plates eventually showed fewer anthrax colonies than 

 similar plates not exposed, the matter was too complicated to give 

 satisfactory results. Obviously one objection was ^that the spores 

 might Lave begun to germinate, and the young colonies killed by the 

 high temperatures. 



Experiments made in October with gelatine plates wrapped in 

 black paper, in which a figure a square, cross, or letter was cut, 

 also led to results too indefinite for satisfaction, although it was clear 

 in some cases that if the plates lay quite flat, the illuminated area 

 was on the whole clear of colonies, while that part of the plate 

 covered by the paper was full of colonies. 



Bat another source of vexation arose. After the plates had been 

 exposed to the sunlight for, say, six hours, it was necessary to put 

 them in the incubator (2022 C. was the temperature used) for 

 two days or so, to develop the colonies, and in many cases it was 

 observed that by the time the colonies were sufficiently far advanced 

 to show up clearly, liquefaction had extended so far as to render the 

 figure blurred and doubtful. 



Stencil plates of zinc were employed with, at first, equally un- 

 certain results. The stencil plate was fixed to the bottom of the plate 

 culture, outside, and every other part covered with blackened paper : 

 the plate was then placed on a level surface, the stencil-covered face 

 upward, and exposed to the direct sunlight. As before, the gelatine 

 softened and in many cases ran, and the results were uncertain, 

 though not altogether discouraging. 



In November it was found that more definite results could be 

 obtained, and the problem was at last solved. 



Meanwhile it had already been found possible to obtain sun prints 

 in the following way with agar plates. Ordinary agar was heated 

 and allowed to cool to between 50 and 60 C., and was then richly in- 

 fected with anthrax spores, and made into plates as usual. Such 

 plates were then covered with a stencil plate on the lower face the 

 stencil plate being therefore separated from the infected agar only 

 by the glass of the plate and wrapped elsewhere closely in dull 

 black paper, so that, on exposure to the sun, only the cut-out figure 

 or letter allowed the solar rays to reach the agar. 



Such plates were then exposed to the direct rays of the October 

 sun for from two to six hours; or they were placed on the ring of a 

 retort-stand, stencil downwards, and the sun-light reflected upwards 

 from a plane mirror below. 



After the insolation, these plates were incubated for at least forty- 



