Art. LIII.—An Optical Illusion.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 33, 1900, Page 434

Art. LIII.—An Optical Illusion. By G. W. Tiffen. [Read before the Hawke's Bay Philosophical Institute, 10th September, 1900.] Some time in March last the following clipping appeared in the Poverty Bay Herald:— “A friend, known to us as trustworthy, sends us the following account by a relative of hers of a remarkable phenomenon of light which she recently observed at Gisborne. She says, ‘As we were coming up the hill on the Whatau-poko side of the footbridge, the sun, which was just setting, being at our backs, we, of course, saw our shadows walking up the hill in front of us. But, instead of being black, as they should have been, our dresses were white, with a very faint black rim round the hem and up the sides, and were perfectly transparent. Now, I had on my last winter dress, which is long and very thick indeed, and Lu had on a navy-blue serge; but, in spite of that, we could see through skirts, petticoats, and everything else, our limbs, black, shapeless, and exaggerated in thinness, reaching right from our shoes to our waists. We were so astonished that we thought we must have been mistaken, and went away back to the bridge and walked up the hill again, with exactly the same result. Did you ever hear anything so queer? It was a very stuffy, close day, and just at that precise time there was a rainbow over Kaiti Hill, which was also peculiar, being thick in the middle and thin at both ends, and with some of its colours bright and others very dull. As soon as we reached the top of the hill our shadows became black again, and continued so till the sun had set.’ Will some experienced scientific friend explain this affair for the benefit of our readers? Mr. J. L. Holland writes to the Auckland Herald, stating that the same atmospheric effect was discovered by members of his family two years ago in front of his own residence, Grafton Road. The conditions under which it has been observed are: The pavement thoroughly wet and the night fairly dark, with the gas-lamp on the opposite corner alight, the shadow of a person thrown on the fence in front of the house gives the exact result described by the ladies in the paragraph—namely, a modification of the Rontgen rays, the limbs being distinctly seen through the dresses or clothing.” The nature of the above-mentioned clipping is so very startling that the reader's first thought is one of incredulity,

while the suggestion that the effects produced were somewhat akin to those of the Rongten rays causes the reader to look upon the discovery with some degree of interest. It was with some such feelings as these that I decided, if possible, to see through myself. Before doing so it was necessary to ascertain what were the necessary conditions. Now, from the clipping it will be found that the phenomenon was observed, by different persons and at different times, in both Gisborne and Auckland. On comparing them we find that the conditions in both cases were somewhat similar. Thus, at Gisborne we find—(1) The sun at a low angle and behind the observers; (2) a sheet of water, also behind, but below the observers; (3) an incline plane on which the shadow was cast. At Auckland the conditions were—(1) A gas-lamp, apparently at some distance away and consequently at a low angle; (2) water over the pavement and below the observer; (3) a fence on which the shadow was cast. One other point may be noticed—namely, at Gisborne there was a rainbow. As no useful results were obtained by the use of a prism, it would appear that the rainbow is not in any way associated with the observed phenomenon, and this is borne out by experiment. Having ascertained the necessary conditions, we are now in a position to experiment on ourselves; but before proceeding to do this let us notice some peculiarities respecting the shadow. We learn—(1) That, instead of being black, it was “white, with a faint black rim round the hem and up the sides”; transparent, or semi-transparent, more aptly describes its appearance: the dresses themselves were not white or transparent, as may be seen by experiment. (2.) The shadow of the limbs reached as high as the waist, but no higher, nor can it be produced higher than the waist. (3.) The phenomenon was lost when the observers reached the top of the hill. (The hill here spoken of is merely the steep eastern bank of the river; on the western side the bank has a very gradual slope towards the river.) Now, on crossing over the bridge from west to east it will be found that the path bends slightly towards the right hand as it ascends the bank of the river, thus allowing the sun's rays to be uninterruptedly reflected from the river. This reflection would, at that time of day, nearly coincide with the slope of this bank, and therefore the reflected rays would not be met with after the top of the bank was reached. As the phenomenon may be seen as well by night as by day, it is optional whether we investigate it by either sunlight or lamplight. Lamplight offers perhaps the best advantages, though the shadow is necessarily less distinct than that produced by daylight.

My experiments were carried out at night. They were simple. The asphalted yard was thoroughly wetted and the kitchen gas-jet lighted. This threw its light both on the asphalt and on a paling fence. On putting on my overcoat and going close to the fence I was greatly astonished to find, in the shadow, my lower limbs apparently as high as my waist. So far, then, all was well, the newly discovered species of Rontgen ray acting beautifully; but, alas! when my overcoat touched the ground, preventing light passing between it and the asphalt, the phenomenon vanished. At the same time the shadow lost its transparency and became black. On the coat, which is perfectly opaque, being raised the phenomenon reappeared. On holding the coat out with one hand and with the other placing my hat behind the coat, nothing of the hat could be seen in the shadow, but when the hat was lowered until its rim was below the edge of the overcoat that part of the hat was seen higher up in the shadow, appearing like a whole hat. On another occasion, using a piece of board and a stick, two distinct shadows were cast, showing the stick not joined, but well separated. It appears, therefore, to be pretty clear that the light does not pass through the clothing, and that the phenomenon is caused by reflected light blending with the original shadow and casting an elongated shadow of that part of one's lower limbs below the dress or cloak into the original shadow, thus producing the appearances already described. This “strange freak of light” may therefore be well classed among the optical illusions.