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1. In dark adaptation the threshold is raised as though the bleaching of visual pigment generated an equivalent background. Now Stiles has shown that real coloured backgrounds act selectively upon the various colour mechanisms, so we ask: `Do equivalent backgrounds from coloured bleachings also act selectively?'

2. When dark adaptation was plotted using a blue test flash (Fig. 1b) following bleaching by orange light a kinked curve was obtained. The upper branch was shown to have the same dark adapted threshold as Stiles blue (₁) mechanism and the lower branch as his green (₄) mechanism. The ₄ dark adaptation curve alone (unkinked) was obtained using a white instead of an orange bleach.

3. Dark adaptation curves were obtained in which the test flash was presented upon various steady backgrounds. In conditions where only ₄ was involved (Fig. 3) the experimental results fitted the curves calculated on the assumption that the equivalent background of bleaching simply adds to the real background in raising the threshold—a condition already established for rods.

4. In conditions where ₄ and ₁ were both present (blue test, yellow-green background and white bleach) kinked dark adaptation curves were obtained (Fig. 2) where the upper branch (₄) coincided with those of Fig. 3 and the lower were due to ₁.

5. The blue mechanism recovers in dark adaptation at about the same rate as red and green, or slightly slower.

6. Dark adaptation curves with red (₅) and green (₄) limbs can be obtained after a deep red bleach (Fig. 4) using a red test flash and a green background. The red and the green limbs were also plotted alone in their entirety by slightly changing the conditions.

7. We are led to the idea of three colour mechanisms that adapt as independently one of another after bleaching as they do with backgrounds.

8. Though this simple independence accounts for the wide and conspicuous range of adaptive phenomena, we have encountered some special conditions (not here described) that seem to imply a measure of interaction between the different colour mechanisms.