BUT VEKY SENSITIVE TO ULTRA-VIOLET RAYS. 207 



rays ; but, on the other hand, that they are very sen- 

 sitive to the ultra-violet rays, which our eyes cannot 

 perceive. 



I then arranged the same ants in a wooden frame 

 consisting of a base and two side walls, between which 

 in the middle was a perpendicular sliding door. The 

 pupae had been arranged by the ants in the centre of 

 the nest, so that some were on each side of the door. 

 We then threw, by means of a strong induction-coil, a 

 magnesium-spark on the nest from one side, and the 

 light from a sodium-flame in a Bunsen burner on the 

 other, the light being in each case stopped by the sliding 

 door, which was pressed close down on the nest. In this 

 way the first half was illuminated by the one light, the 

 second by the other, the apparatus being so arranged 

 that the lights were equal to our eyes — that, however, 

 given by the magnesium, consisting mainly of blue, 

 violet, and ultra-violet rays, that of the sodium being 

 very yellow and poor in chemical rays. In a quarter of 

 an hom* the pupae were all carried into the yellow. 

 The sodium light being the hotter of the two, to 

 eliminate the action of heat I introduced a water- cell 

 between the ants and the sodium-flame, and made the 

 two sides as nearly as possible equally light to my eye. 

 The pupre, however, were again carried into the sodium 

 side. 



I repeated the same experiment as before, getting 

 the magnesium- spark and the sodium-flame to the same 

 degree of intensity, as nearly as my eye could judge, 



