1. The amount of visible or long ultraviolet light (UV) required to photoreactivate Colpidium colpoda injured with known dosages of short UV (2654 A) was determined.

2. The effect of the short UV was tested by the delay in division of exposed animals compared to controls. Photoreactivation was tested by the effect of postillumination on the delay of division of treated colpidia compared to controls.

3. Colpidia were used in two physiological states: well fed and starved in balanced medium for 48 hours. The latter are much more sensitive to short UV although less susceptible of photoreactivation.

4. Photoreactivation occurred over the entire span from 3350 A to 4350 A for the well fed colpidia, from 3130 A to 5490 (green) for starved colpidia.

5. The photoreactivating effect of a single quantum of blue (4350 A) or long UV (3660 A) delivered per quantum of 2654 A used to injure colpidia was too slight to be considered significant. The effect of 10 quanta was usually more pronounced, but only after 100 quanta had been delivered was the photoreactivation nearly maximal for well fed colpidia.

6. The quantum requirement for maximal photoreactivation of the starved animals was greater at all wave lengths tried: 3660, 4050, 4350, and 5460 A being of the order of 800 incident quanta per incident quantum of 2654 A.

7. The transmission of UV(2654 A), blue, yellow, and red light by a suspension of colpidia was determined.

8. Large dosages of blue, violet, or long UV were slightly injurious to starved colpidia. In a few cases large dosages of 3660 A killed starved colpidia, especially after a non-lethal dose of short UV(2654 A).

9. Photoreactivation seems to be a balance between the slight injurious effect produced by the visible light or UV of long wave lengths and the injury produced by short wave length UV.

10. Possible reasons for the large number of quanta of photoreactivating light required per quantum of short UV are discussed.

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