1G70 THE BUTTERFLIES OF NEW ENGLAND. 



received much uew light from the anatomical investigations of Patten and 

 others and the ex[)erimental researches of Forel and Plateau. The com- 

 pound eyes of butterflies, as of other insects, are made up externally of a 

 number of adjoining hexagonal facets, each separate facet being the ex- 

 posed portion of a crystalline lens which is followed beneath by a slender 

 tube containing, first, a terminal body, the crystalline cone or retinidium, 

 formerly looked upon as a second refracting medium, but by Patten re- 

 garded as a retinal body sensitive to the light : and, second, of a collec- 

 tion of rods. The office of this so-called crystalline cone is the principal 

 point in dispute. The later researches regard it as a receptacle for the ter- 

 mination of the nervous elements, and as performing no office in either 

 modifying or destroying the image ; while the old view regarded it as 

 having similar properties to the crystalline lens of our own eye. Now 

 if the later view be regarded as correct, the form and nature of this recep- 

 tacle is such, as Plateau points out, that an image may be formed at any 

 point within its depth, but that at the same time all the sensitive points of 

 the cone in advance of or behind an image will be illuminated and will 

 also to some degree be excited by the same object, so that whatever image 

 is formed can in no w-ay be seen as distinct but only as entirely confused ; 

 much as happens in the human eye when the image is focussed beyond the 

 retina. 



This theoretical view can be perfectly well subjected to experiment, and 

 this has been done in the most thorough manner by Plateau. At first his 

 experiments were made almost entirely by placing the insect desired to be 

 experimented upon at one end of a closed compartment at whose other end 

 were two distinct openings to the light, one simple and large enough for the 

 escape of the insect, the other much larger but covered with a trellis or 

 grating, forbidding passage ; so that while the actual superficial area of the 

 open spaces might be the same in each case, admitting the same quantity 

 of light, the trellised opening would appear greater. The insects almost 

 invariably flew to the latter. At the same time one could modify at will 

 the amount of light which would enter either of these two diffiirent classes 

 of openings. Numerous experiments were made by Plateau upon this 

 basis, resulting in his conclusion that insects with compound eyes did not 

 well distinguish between two illuminated openings, being sometimes led 

 astray by the excess of luminosity, sometimes by the apparent excess of 

 surface. In general they could not distinguish the form of objects or 

 only in a vague way. 



Objections were raised that in these cases it was not objects but 

 luminous openings, the power of seeing the form of which was tested ; and 

 also that the judgment of the insect was brought into service under un- 

 natural conditions, so that the experiments proved nothing definite and 

 decisive regarding the actual power of visibility on its part. There- 

 nj)on Plateau devised a new method by which experiments could be 



