RESISTANCE TO LETHAL AGENTS 309 



5. Expose these plates to the action of diffuse day- 

 light (not direct sunlight) in the laboratory for one, 

 two, three, four, five, six, eight, ten, twelve hours. 



6. After exposure to light, incubate under optimum 

 conditions. 



7. Examine the plate cultivations after twenty-four 

 and forty-eight hours' incubation, and compare with 

 the two controls. Record results. If growth is absent 

 from that portion of the plate unprotected by the 

 black paper, continue the incubation and daily obser- 

 vation until the end of seven days. 



8. Control the results. 



(b) Direct Sunlight : 



1. Prepare plate cultivations precisely as in the 

 former experiments and place the two controls in the 

 incubators. 



2. Arrange the remaining plates upon a platform in 

 the direct rays of the sun. 



3. On the top of each plate stand a small glass dish 

 14 cm. in diameter and 5 cm. deep. 



4. Fill a solution of potash alum (2 per cent, in dis- 

 tilled water) into each dish to the depth of 2 cm. to 

 absorb the heat of the sun's rays and so eliminate possi- 

 ble effects of temperature on. the cultivations. 



5. After exposures for periods similar to those em- 

 ployed in the preceding experiment, incubate and 

 complete the observation as above. 



(c) Primary Colours: Each colour violet, blue, 

 green and red must be tested separately. 



1. Prepare plate cultivations, as in the previous 

 "light" experiments, and incubate controls. 



2. Fasten a strip of black paper, 3 cm. wide, across 

 one diameter of the cover of each plate. 



3. Coat the remainder of the surface of the cover 

 with a film of pure photographic collodion which con- 

 tains 2 per cent, of either of the following aniline dyes, 

 as may be necessary. 



