OF ARTS AND SCIENCES. 247 



colors to enter, while they reflect all others. Such bodies show one 

 color when the light falls upon them, and another when the light is 

 transmitted through them. The metals in very thin leaves are espe- 

 cially prominent in this class of bodies. The various aniline pigments 

 also show such surface colors when dry. Thin layers of those pig- 

 ments will have a perfectly metallic appearance. Fuchsine or Ma- 

 genta has a green golden color when the light falls upon it, a purple 

 color when the light is transmitted through it." 



The surface colors are prominently interesting for entomology, as 

 they explain for the first time the frequent metallic colors on insects, 

 which have often so true a metallic appearance, that indeed it has been 

 tried to extract the gold out of them. The condition that such layers 

 must be perfectly dry, makes it evident, as I have before stated, that 

 metallic colors must belong to the dermal ones. As I have stated 

 later, that insect colors might with propriety be compared with aniline 

 colors, the whole comparison is even more to the point. 



4. ^^Fluorescent colors [Bezold, p. 621] act quite differently from the 

 before-mentioned ones. In all those cases a division of the impin- 

 ging rays took place, so that a part of the rays entered the body or 

 passed through it, and that the remainder was absorbed or reflected. 

 But the fluorescent bodies transform the light falling upon them into 

 light of another color, that is to say of different wave-length. This 

 is frequently observed in the yellowish-green uranium glass, in fluor- 

 spar, in solution of quinine, in petroleum and other bodies. Investi- 

 gations with the aid of the spectru-m show that in the case of bodies 

 of this kind light of the most varied colors is transformed into lio[ht 

 of some other definite color, and that even invisible rays can be 

 changed into visible ones, as the ultra-violet rays. The retina pos- 

 sesses in a slight degree the property of fluorescence." I jjresume 

 that a number of insect colors may probably belong to this kind, 

 namely, the violet shades observed in Rutela and similar beetles. 



■I have i^urposely dwelt at length on these phenomena, as I believe 

 that they will throw considerable light ui^on occurrences till now 

 unexplained. 



Hypodermal and Dermal Colors. 



The hypodermal colors are very often different in males and females 

 of the same species. The dermal colors rarely differ, so far as I 

 know, in the sexes. But there are some genera with prominent 

 dermal colors, which are nearly always different in both sexes, as 

 Calopteryx, some Ilymenoptera, some Coleoptera, and others. 



