The purified visual pigments of the cat, horse, squirrel, deer, and guinea pig have lambdamax values at 553, 545, 532, 531, and 516 nm, respectively, which are precise to within +/-1 nm. These opsins were expressed in COS1 cells and reconstituted with 11-cis-retinal. To elucidate the molecular mechanisms of red-green color vision in mammals, we have cloned and sequenced the red and green opsin cDNAs of cat (Felis catus), horse (Equus caballus), gray squirrel (Sciurus carolinensis), white-tailed deer (Odocoileus virginianus), and guinea pig (Cavia porcellus). The reasons for the radiation and diversity of butterfly colour vision remain unknown, but may include their need to find food plants and to select mates. Butterflies are unusual in that they have enjoyed a major radiation in receptor numbers and spectral sensitivities. On the other hand, the colours of fruit, flowers and feathers may have evolved to be more discriminable for the colour vision of their natural receivers than for other groups of animals. Although the spectral sensitivities of photoreceptors are known to vary adaptively under natural selection there is little evidence that those of hymenopterans, birds and primates are specifically adapted to the reflectance spectra of food plants or animal visual signals. This article discusses the evolutionary relationship between photoreceptor spectral sensitivities of four groups of land animals-birds, butterflies, primates and hymenopteran insects (bees and wasps)-, the colour signals that are relevant to them, and how understanding is informed by models of spectral coding and colour vision. The visual displays of animals and plants are often colourful, and colour vision allows animals to respond to these signals as they forage for food, choose mates and so-forth. Because of the large number of paintings studied, we tentatively propose the conclusion that regardless of the school, red-to-green ratios from great masters can provide independent proxy AODs that correlate with widely accepted proxies and with independent measurements. However, a detailed study on all possible sources of uncertainties involved (such as the impact of clouds on R / G ratios) still needs to be studied. Also, when analysing different parts of cloudless skies of paintings following major volcanic eruptions, any structural differences seen in the paintings had not altered the results discussed above.
It was found that the differences in terms of R / G ratios were small, ranging between -2.6% and +1.6%. An independent experiment was performed to understand the difference between R / G ratios calculated from a typical volcanic aerosol and those measured from the mineral aerosol during the Hydra experiment. Independent solar radiometric measurements confirmed that the master colourist's R / G ratios which were used to model his AODs, matched the AOD values measured in situ by co-located sun photometers during the declining phase of the Saharan aerosol.
To corroborate our findings, an experiment was designed in which a master painter/colourist painted successive sunsets during and after the passage of Saharan aerosols over the island of Hydra in Greece.
The increase of AOD at 550 nm calculated from the paintings grows from 0.15 in the middle 19th century to about 0.20 by the end of the 20th century. The remaining "non-volcanic" years were used to provide additional evidence of a multidecadal increase in the atmospheric optical depths during the industrial "revolution". The year of each eruption and the 3 following years were defined as "volcanic". AODs calculated from paintings were grouped into 50-year intervals from 1500 to 2000. Aerosol optical depths (AODs) at 550 nm, corresponding to Northern Hemisphere middle latitudes, calculated by introducing red-to-green (R / G) ratios from a large number of paintings to a radiative transfer model, were significantly correlated with independent proxies from stratospheric AOD and optical extinction data, the dust veil index, and ice core volcanic indices. Images derived from precision colour protocols applied to the paintings were compared to online images, and found that the latter, previously analysed, provide accurate information.
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