3. Place one agar plate and one gelatine plate into the hot and cold incubators, respectively, as _controls_. 4. Fasten a piece of black paper, cut the shape of a cross or star, on the centre of the cover of each of the remaining plates (Fig. 159). 5. Expose these plates to the action of diffuse daylight (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 observation 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 distilled water) into each dish to the depth of 2 cm. to absorb the heat of the sun's rays and so eliminate possible effects of temperature on the cultivations. 5. After exposures for periods similar to those employed 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 contains 2 per cent. of either of the following aniline dyes, as may be necessary: Chrysoidin (for red). Malachite green (for green). Eosin, bluish (for blue). Methyl violet (for violet). 4. Expose the plates, thus prepared, to bright daylight (but not direct sunlight) for varying periods, and complete the observations as in the preceding experiments. The bactericidal action of light appears to depend upon the more refrangible rays of the violet end of the spectrum and is noted whether the red yellow rays are transmitted or not. 5. Control the results. NOTE.--The ultra-violet rays obtained from a quartz mercury vapour lamp destroy bacterial life with great rapidity under laboratory conditions. (C) _Heat._--(_Vide_ Thermal Death-point, page 298.) (D) _Antiseptics and Disinfectants._--The resistance exhibited by any given bacterium toward any specified disinfectant or germicide should be investigated with reference to the following points: