226 Discovei-ies of Miiller and others in the 



The cornea (<z) is thickest at the posterior part of the eye ; 

 the facets there being about four times as thick as they are 

 broad ; in that part also it may be readily seen that each facet 

 is separated from the adjoining ones by an opaque line, a kind 

 of suture, which gives to the whole of this transparent layer a 

 bluish tinge, and thus softens, when the eye is examined ex- 

 ternally, the intense colour of the pigment beneath. 



The black pigment {c) forms a layer of a very dark colour ; 

 but its thickness is not so great as that of the cornea. MUUer 

 very justly regards it as identical with the pigment situated 

 more deeply in the eye, and which will be mentioned here- 

 after. At a first and cursory examination it might very 

 readily be supposed that this layer is perfectly continuous 

 beneath the cornea, so as to intercept completely the passage 

 of light to the parts within it ; but a careful removal of the 

 internal structures of the eye, leaving this pigment untouched, 

 will show that, although very thick at the sutures of the facets, 

 where it is continuous with the pigment of the more internal 

 textures, it becomes, towards the centre of each facet, exceed- 

 ingly thin, and at the very centre no pigment can be seen ; 

 a minute perforation, as it were, in the layer being there 

 observed. If a cornea, with its layer of pigment still attached 

 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 towards the 

 centre of the eye, are necessarily cut and torn in exposing 

 the internal surface of the cornea ; and consequently they 

 float and waver 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 intercept the 

 passage of light, 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, a priori, be expected, it is not met with in any 

 of the nocturnal insects. 



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 



