Microspectrophotometric measurements have been made of the photopigments of individual rods and cones from the retina of a man. The measuring beam was passed transversely through the isolated outer segments. 2. The mean absorbance spectrum for rods (n = 11) had a peak at 497.6 +/- 3.3 nm and the mean transverse absorbance was 0.035 +/- 0.007. 3. Three classes of cones were identified. The long-wave cones ('red' cones) had a lambda max of 562.8 +/- 4.7 nm (n = 19) with a mean transverse absorbance of 0.027 +/- 0.005. The middle-wave cones ('green' cones) had a lambda max of 533.8 +/- 3.7 nm (n = 11) with a mean transverse absorbance of 0.032 +/- 0.007. The short-wave cones ('blue' cones) had a lambda max of 420.3 +/- 4.7 nm (n = 3) with a mean transverse absorbance of 0.037 +/- 0.011. 4. If assumptions are made about the length of cones and about pre-receptoral absorption, it is possible to derive psychophysical sensitivities for the cones that closely resemble the appropriate pi mechanisms of W. S. Stiles. 5. If assumptions are made about the length of rods and about pre-receptoral absorption, however, the psychophysical sensitivity derived for the rods is considerably broader than the C.I.E. scotopic sensitivity function.
