170 



SCIENCE. 



[N. S. Vol. I. No. 7. 



Photobacteriographs were made by Buch- 

 ner's method, namely, by placing a square 

 of black paper, or of glass of different colors, 

 upon the bottom of a plate containing in- 

 oculated agar-agar during insolation; but 

 although the protected portion was visible 

 after fifteen minutes' insolation and incuba- 

 tion for twenty-four hours, and sharplj^ de- 

 fined after two hours' insolation and incuba- 

 tion as before, no accurate estimate could 

 be made of the difference in the destruc- 

 tive power of different periods of inso- 

 lation. Successfal photobacteriography re- 

 quires inoculation of large quantities of 

 bacteria, in order that the colonies may be 

 set so closely together that a ground-glass 

 appearance is produced; in which case 

 coiinting of the colonies is practically im- 

 possible. 



For this reason the following method was 

 used for each of the three organisms, 

 the staphylococcus pyogenes aureus, the bacillus 

 coli cammunis and the bacillus typhi abdom- 



To obtain an accurate measure of the 

 effects produced by lights of different in- 

 tensity or of different colors, it is necessary 

 to ensure, as far as possible, that the bac- 

 teria to be experimented on shall be uni- 

 formly distributed in the culture media. 

 Tubes containing each 10 cc. of bouillon 

 were inoculated with one drop of a bouillon 

 ciilture and then placed in an incubator for 

 twenty-four hours. A small quantity of 

 sterilized gravel was then added to the 

 culture tube and it was thoroughly shaken, 

 after which 10 cc. of a one-half per cent, 

 salt solution was added and the culture 

 drawn into a Nuttall's dropping apparatus. 

 From this, one-twentieth of 1 cc. of the 

 bouillon culture was dropped into a tube of 

 melted agar-agar, which was slowly and 

 thoroughly agitated, and the contents were 

 then poured into a Petri dish, carefully 

 levelled on a levelling tripod over ice water. 

 In the first method used the Petri dishes 



were found to be so uneven on the bottom 

 that the layer of medium under the pro- 

 tective square was often very thick or very 

 thin as compared with that about the cir- 

 cumference of the plate, and, therefore, 

 comparisons made between the centre and 

 the cu'cumference would be in almost every 

 case unreliable. To overcome this diffi- 

 cultj', just one-half of the plate was shaded 

 mth black paper or colored glass. 



The plates were then exposed to sunlight, 

 bottom upwards, so as to allow the sun to 

 shine as directlj^ as possible on the inocu- 

 lated agar-agar. At intervals of fifteen 

 mhiutes a plate was removed and placed in 

 the incubator. The temperature of the 

 plates during insolation was always 

 below 34° C. as shown by a thermometer 

 with a blackened bulb which was placed in 

 the sun and the temperature noted every 

 fifteen minutes. Sunny, still days were 

 latilized for insolation, beginning at 10 A. m. 

 during the months of October, November 

 and December. After insolation, the plates, 

 and also a non-insolated control plate were 

 incubated for tn^enty-four hours. 



The colonies were counted in the follow- 

 ing manner : A number 1 eye-piece was 

 divided into fields (as done by Nuttall in 

 counting tubercle bacilli), by inti'oducing a 

 disk of black cardboard which had a square 

 opening divided into four parts by two hairs 

 placed at right angles. This eye-piece and 

 an objective of low power were used in 

 counting. The percentage of germs de- 

 stroyed by insolation was estimated &"om 

 the mean of four counts taken on both the 

 insolated and the protected halves of the 

 plate. Bjr this method an accurate state- 

 ment can be made regarding the difference 

 in protective power given by the different 

 colors, not from simple observation, but by 

 comparison of a definite number of colonies 

 counted. 



The following table shows the compara- 

 tive effect of the blue raj'^s and of complete 



