VII] THE SENSE ORGANS 149 



change the form of energy which we call light into that other 

 form known as nervous energy. In this form, the theory has 

 found wide acceptance. 



Exner [53] propounded what amounts to a modification of the 

 Mosaic Theory. Through a freshly cut-out compound eye of a 

 Fire-fly, he obtained a photomicrograph of a distant church 

 steeple, together with a window-pane on which the letter R had 

 been pasted in black paper. The image was erect and complete. 

 He explained this by supposing that the images formed by the 

 separate ommatidia overlap and merge,, so as to give the impression 

 of a complete whole. He termed these "superposition" images, 

 as opposed to non-overlapping or "apposition" images. It can 

 be easily understood that an alteration in the position of the 

 pigment layers might convert a set of apposition images (forming 

 a mosaic) into a continuous image, by admitting just enough 

 light to cause the boundaries to merge, and so obliterate the 

 darkened borders of the mosaic pieces in a hexagonal pattern. 

 It is probable that all insects with strong visual powers can modify 

 the position of their pigment layers sufficiently to allow of such 

 accommodation in various degrees of light-intensity. 



The Theory of Dioptric Vision was enunciated by Lowne[86] 

 in 1884. It differs entirely from the Mosaic Theory, both as to 

 the method of vision and the limits of the eye. The supporters 

 of the Mosaic Theory consider the fenestrate membrane to be the 

 inner limit of the eye proper. Thus the corneal lens and crystalline 

 cone form the dioptric portion of the eye, while the retinulae are 

 the receptive portion or retina, in direct communication with the 

 optic ganglion. All structures lying below the fenestrate membrane 

 are considered to belong to the optic ganglion. But, according to 

 Lowne, the whole of the portion lying above the fenestrate mem- 

 brane is dioptric in function, while the true retina is the pigmented 

 portion lying below the fenestrate membrane. The rhabdomes 

 must then act as another set of lenses, in such a manner that all 

 the rays of light received by the whole eye are brought to a focus 

 on the retina, yielding a complete image like that formed by the 

 vertebrate eye. This image would be an erect one. Hickson [73] 

 supports Lowne's view of the position of the retina. 



