DIE SICHTBARKEIT ULTRAVIOLETTEN LICHTES
Authors: MERKER E. 
Published: January 2008

1. Ultra‐violet light produces a strong bluish white fluorescence in the eyes of all vertebrate animals hitherto investigated (mammals, birds, reptiles, and amphibians). The lenses shine most strongly but to varying extents, those of young animals glowing less than the lenses of older individuals. It appears that twilight animals have less strongly fluorescent lenses than diurnal forms. The transparency of the component parts of the eye decreases in proportion to the amount of fluorescence, without, however, being completely abolished. Vertebrates can barely see wave lengths shorter than 300 mμ, and many lenses completely absorb light below 313 mμ (frog, cat). Ultra‐violet images on the human retina may be suppressed by fluorescence from the lens. The brightness of ultra‐violet light is small, and for this reason it does not disturb us in daylight. Inflammation of the front parts of the eye may result from the shorter ultra‐violet rays of the sun. Normally the retina and the floor of the eye are protected by the strong absorption of these rays in the lens. Nevertheless, strong ultra‐violet light may injure the retina.

2. In minnows a movement of the retinal pigment occurs even in the long wave ultra‐violet, accompanied by a migration of rods and cones. In pure ultra‐violet the cones are situated on the limiting membrane, as they are in visible light. Fluorescent light produces the same effect, although less strongly than ultra‐violet.

3. Fluorescence likewise results in the eyes of insects wherever ultra‐violet light falls on colourless tissues. The colourless chitin over the eye shines strongly when it is sufficiently thick. Nevertheless, ultra‐violet light cannot be demonstrated at the back of the eye. The tracheal tapetum of many eyes shines noticeably, and by the disappearance of the glow it can be recognised that this eye pigment too migrates in ultra‐violet light. In the case of sensitive twilight butterflies fluorescent light can produce this movement of pigment, but in diurnal forms ultra‐violet radiation alone is effective.

4. Bertholf showed that bees and Drosophila are attracted to the ultra‐violet spectrum when they are exposed simultaneously to a certain intensity of white light. The strongest maximum is at 366 mμ and is 4.5–5.5 times as strong as that in the green region. Drosophila has 3 maxima, one at 487 mμ, the highest at 366 mμ and a weak maximum at 254 mμ. The bee has 2 maxima only, at 555 mμ and 366 mμ. The bees' spectrum extends further into the red and ends in the short wave region at 300 mμ. The Drosophila spectrum extends beyond 300 mμ but appears to be curtailed in the red.

5. Daphnia turns on to its back in long wave ultra‐violet coming from below, just as it does in visible light, even when exposed at the same time to fluorescent light from above. This demonstrates the stronger effect of ultra‐violet light on the animal.

6. Planarians move away from long wave ultra‐violet just as they do from visible light. Fluorescent light has a weaker action. A physiological equilibrium is attained with a visible light of 200–300 lux opposed to 0.07 B.‐R. units of ultra‐violet. Left‐eyed planarians move away from ultra‐violet light to the right, right‐eyed animals to the left, while those with both eyes move away along a very narrow track. Lighted from above the left‐eyed animals circle to the right, those with right eyes the left. The circles are smaller in strong than in weak light. It follows that planarians can see ultra‐violet light.