VISION IN INSECTS. 135 



to it, be put in water, and its internal surface be then 

 examined at different angles, and with a powerful 

 magnifier, a position will be soon found in which the 

 light will be seen to traverse, without obstruction, 

 the centre of each facet. This position, of course, 

 varies very much, because the internal prolongations 

 or septa, which the pigment forms in its course to- 

 wards the centre of the eye, are necessarily cut and 

 torn in exposing the internal surface of the cornea ; 

 and consequently they float and waiver about in all 

 directions. If the cornea is examined out of water, 

 these septa lie flat upon its internal surface, and, 

 masking in this manner the perforations, exhibit the 

 appearance of one continuous layer. 



' The presence of this layer does not therefore inter- 

 cept the passage of hght, but merely diminishes its 

 quantity. It is found in many, if not all, the diurnal 

 insects, and is perforated with as many holes as there 

 are facets on the cornea ; but, as might indeed be ex- 

 pected, it is not met with in any of the nocturnal in- 

 sects. 



' The zone (e) which is observed in the section of 

 the eye, within the layer just described, is seen, when 

 examined with a powerful magnifier, to be very 

 evidently composed of straight and transparent 

 cylinders, smaller at the lower and anterior part of 

 the eye, where the facets have the least dimensions, 

 than at the upper and posterior part. They are equal 

 in number to the facets of the cornea. The orange 

 and black tints already mentioned are owing to the 

 coloured pigment which extends between these crys- 

 talline cylinders, surrounding and insulating them 

 throughout their whole length. Besides the general 

 difference in size just mentioned, the cylinders are 

 found to be much longer at the back than in the 

 front of the eye ; all are perpendicular to the surface 

 of the cornea, and they converge regularly towards 



