Tiffen. — On an Optical Illusion. 435 



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 neces- 

 sary to ascertain what were the necessary conditions. 



Now, from the clipping it will be found that the phe- 

 nomenon 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 be- 

 hind, 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 con- 

 sequently 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 proceed- 

 ing 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 de- 

 scribes 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 re- 

 flection 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 pro- 

 duced by daylight. 



