THE ACTION OF LIGHT, ETC. 



367 



feld and Mr. Cowper Temple, there will soon be 

 women-doctors, and women's hospitals attended 

 by women-doctors, in every town in the kingdom. 

 I rejoice to know that we possess already a few 

 qualified ladies who every day, without wound to 

 the feelings of the most sensitive, receive the full 

 and free confidence of girls and women, and give 



in return counsels to which many attribute the 

 preservation of life and health ; and which — if 

 medical science have any practical value — must 

 afford the rising generation a better chance than 

 ever their mothers have had of escaping the end- 

 less miseries to themselves and all belonging to 

 them attendant on the Little Health of Ladies. 

 — Contemporary Review. 



THE ACTION OF LIGHT UPON THE COLORATION OF 



THE ORGANIC WORLD. 



UNTIL the earlier portion of the present cen- 

 tury, light, by the vast majority of civil- 

 ized persons, was regarded as a medium for the 

 sense of sight, and as very little more. With 

 the discovery of its chemical functions, brought 

 home to the popular mind by the invention of 

 photography, a revolution in opinion took place, 

 and the danger now is, not that its real powers 

 should be overlooked, but that it should be cred- 

 ited with effects in which its part is very doubt- 

 ful. It has been especially proclaimed to be at 

 once the creator and the destroyer of coloration 

 in the organic world. The superior intensity of 

 the light to which they are exposed has been 

 pronounced the chief cause why diurnal species 

 are more gayly colored than their nearest noctur- 

 nal allies, and why the flora and the fauna — es- 

 pecially the insects and the birds — of tropical 

 regions are so rich in hues of a gorgeous char- 

 acter. It may, therefore, be not uninteresting to 

 inquire into this supposed double function of 

 light, and ascertain, if possible, its limits in 

 either direction. In so doing it will be impossi- 

 ble for us to overlook the views put forward by 

 Mr. A. R. Wallace in a recent issue of Macmillarts 

 Magazine. 



The bleaching power of the sun's rays, and 

 to a less extent of ordinary diffused daylight, 

 has been fully recognized in the affairs of daily 

 life. It has been observed that this same agency, 

 utilized formerly in preparing vegetable fibre for 

 the reception of colors, gradually destroys, in al- 

 most every instance, the work of the dyer and 

 the printer, and exerts a corresponding influence 

 upon the hues of plants. There is, however, a 

 distinction by which its effects upon the integu- 

 ments of animals are limited. 



It is well known that what we designate as 



color may be produced either by the interference 

 or by the absorption of rays of light, and hence 

 the colors of animals may be divided into two 

 well-marked classes. On the one hand, especially 

 in birds and insects, we find hues which are iri- 

 descent, changeable according to the relative po- 

 sitions of the observer and of the light, and are 

 possessed of an intense, so-called, metallic lustre. 

 Such colors — to take familiar examples — may be 

 seen in the plumage of the peacock, of the star- 

 ling, on the wings of the " purple emperor " 

 butterfly (Apatura Iris), on the entire coating 

 of the rose-beetle (Cetonia aurata), of the fire- 

 wasp (Chryseis ignita), and of many other com- 

 mon native insects. In the vegetable kingdom 

 they may be pronounced unknown. Such colors 

 are due to the interference of certain rays of 

 light, whether reflected from superimposed trans- 

 parent films or reflected from or refracted through 

 minute striae. These colors are permanent, even 

 on the most prolonged exposure to air, to atmos- 

 pheric moisture, or to full sunlight. Unless the 

 very texture of the feather, the wing-scale, the 

 elytron, etc., be destroyed by putrefaction or 

 combustion, the color remains unhurt. Nor can 

 we by any means extract from such colored sur- 

 faces a dye or pigment capable of being applied 

 to other objects. 



On the other hand, there are colors which do 

 not change their shade from whatever position 

 they are regarded, and which possess little of 

 that intense lustre which marks the former class. 

 To this kind belong the colors of all flowers, 

 of caterpillars, of the great majority of our na- 

 tive butterflies and moths, and, in short, of the 

 vast bulk of organic beings. These colors are 

 due to the absorption of certain of the rays of 

 light, such absorption being effected by sub- 



