FURTHER REMARKS ON THE COMPOUND EYE. 



twenty-five facets, which accords with the calculated diameter 

 on the view which I have taken of its origin. 



Under all the theories which have been suggested, only very 

 small pencils of light can enter or traverse the great rods, and 

 such pencils could only produce a very feeble reflection, or a 

 very minute one. The minute image so produced can indeed 

 be seen with an ophthalmoscope in the centre of a dark disc, 

 which in most insects replaces the luminous reflection above 

 described. The dark disc is known as a pseudo-pupil, the 

 nature of which will now be discussed. 



The Central or Principal Pseudo-pupil. Exner says [252] : 

 ' I now come to the consideration of a remarkable optical 

 phenomenon, the attempted explanation of which has cost me 

 many a headache. Once more Leydig first observed the 

 phenomenon, but it was scarcely discussed until Thompson 

 Lowne again observed it. Lowne remarked in a butterfly 

 (Pict'is brassicce) six black spots arranged around a central 

 spot, and explains it as a diffraction image.' 



These spots are seen with the naked eye or with a simple 

 lens, and whatever the position of the insect the central spot is 

 exactly opposite the observer's eye. The central spot is the 

 primary or principal pseudo-pupil, and I think it is undoubtedly 

 due to the fact that the light which falls directly upon the 

 cornea is feebly reflected, whilst that which falls on it 

 obliquely is reflected more powerfully. In other words, the 

 cornea in the region of the central pseudo-pupil is seen as a 

 transparent object, and the dark pigment beneath it absorbs 

 the light, whilst the surrounding cornea is seen as an opaque 

 object, and by reflected light. 



If the central pseudo-pupil is observed with an ophthalmo- 

 scope adapted to a microscope, from which the eye-piece 

 has been removed, with a powerful light direct sunlight 

 is best a small central bright spot is seen within the dark 

 pseudo-pupil. This phenomenon is described by Exner. The 

 light by which this bright spot is seen undoubtedly traverses 

 the great rod, and is, I think, reflected by the retinal rods ; 

 by careful focussing, the image of the retinula (in sensu mihi) is 



