132 



NA TURE 



[February 2, 189; 



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 

 uncertain 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 oa,a level surface, 

 the stencil -covered face upward, and exposed to the direct sun- 

 light. As before, the gelatine softened and in many cases ran, 

 and the results were uncertain, though not altogether dis- 

 couraging. 



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 infected 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 separa- 

 ted from the infected agar only by the glass of the plate — and 

 wrapped elsewhere closely in dull black paper, so that, on ex- 

 posure 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 Octo- 

 ber sun for from two to six hours ; or they were placed on the 



ring of a retort-stand, stencil downwards, and the sunlight 

 reflected upwards from a plane mirror below. 



After the insolation these plates were incubated for at least 

 forty-eight hours at 20° C, and on removing the wrappers the 

 colonies of anthrax were found densely covering all parts of the 

 plate except the area — a letter or cross, &c. — exposed to the 

 sunlight. There, however, the spores were killed, and the 

 agar remained perfectly clear, showing the form of a sharp 

 transparent letter, cross, &c., in a groundwork rendered cloudy 

 and opaque by the innumerable colonies of anthrax. 



Experiments proved that this was not due to high tempera- 

 ture, for a thermometer with its bulb next the insolated glass 

 rarely rose beyond 14° to 16° C, and never beyond 18° C, and 

 even if the thermometer did not record the temperature inside 

 the plate, this can scarcely have been much higher. 



As long as this latter point remained uncertain, however, the 

 experiments could not be regarded as satisfactory ; whence it 

 was necessary to again have recourse to gelatine cultures. The 

 gelatine employed began to run at 29° C, and in November it 

 w as found that such plates exposed outside, either to directly 

 incident sunshine, or to directly reflected rays, showed a tem- 



NO. I 2 14, VOL. 47] 



perature of 12° to 13° C. at the insolated glass surface, and even 

 five to six hours exposure caused no running of the gelatine. 



The following experiment may be selected as a type of the 

 rest : — A (Fig. i) is the upright of an ordinary retort-stand ; on 

 the ring B rested a gelatine plate culture of anthrax spores, 

 covered with black paper everywhere except the cut-out letter 

 E, seen on its lower face, c was an ordinary plane microscope- 

 mirror with its arm fitted to a cork on A. 



The whole was placed in the middle of a field at 

 9.30 a.m. on Wednesday, November 30, and exposed to 

 the clear, but low, sunshine which prevailed that day, the 

 mirror being so arranged (from time to time as necessary) as to 

 reflect the light on the E the whole period, until 3.30 p.m., 

 when the plate was removed and placed in the dark incubator 

 at 20° C. On the following Friday— j.<r. after less than forty- 

 eight hours' incubation — the letter E stood out sharp and clearly 

 transparent from the faint grey of the rest of the plate of gela- 

 tine. Not a trace of anthrax could be found in the clear area, 

 even with the microscope, while the grey and almost opaque 

 appearance of the rest of the plate was due to innumerable 

 colonies of that organism which had developed in the interval. 



It was impossible to incubate the plate longer for fear of lique- 

 faction, whence the sceptical may reply that the anthrax exposed 

 to light was only retarded ; the experiments with agar show that 

 such is not the case, however, and that if the insolation is com- 



plete the spores are rendered incapable of germinating at all, 

 as proved by removing pieces of the clear fgar or gelatine and 

 attempting to make tube cultures from them : in all cases where 

 insolation is complete they remain sterile. 



The chief value of these gelatine plate exposures in November, 

 however, is that they prove conclusively (i) that the rays of a 

 winter sun are capable, even if reflected, of killing the spores, 

 and (2) that it is really the solar rays which do this directly, and 

 not any effect of a higher temperature, since the gelatine remains 

 solid throughout. 



Experience has shown, however, that some precautions are 

 necessary in selecting the anthrax cultures employed for these 

 experiments with gelatine. The light certainly retards or kills 

 (accarding to its intensity or the length of exposure) virulent 

 spores, but if one takes the spores, mixed with vegetative bacilli, 

 direct from a thoroughly liquefied gelatine culture, or from a 

 bouillon culture, the plates are apt to be liquefied too rapidly for 

 the proper development of the light print, evidently because so 

 much of the liquefying enzyme is carried in when inoculating 

 the plates. The same danger is run when active bacilli alone 

 are employed. 



The best method of avoiding these disadvantages has been 



