THE COMPOUND EYE 



171 



which clearly shows that the definition of the image depends on the 

 number of tubes per unit area (Alverdes, 1924). Using the excised 

 anterior segment of the eye of the fire-fly, Lampyris, as a lens, Exner 

 (1891) succeeded in photographing the image (Fig. 157) ; the degree 

 of resolution thus obtained has been estimated by Marchal (1910) to 



Fig. 156. — The Compound Eye According to Johannes Muller. 



When light emitted by different points, a, b, c, d, falls on the ej^e, that 

 from a completely illuminates cone e, but the ommatidia to the right of e 

 are not illuminated all the way down. Only the nerve /, issuing from cone e, 

 is thus stimulated by the source a, while light from the same source entering 

 other onimatidia is unable to stimulate the fibres since it is absorbed by 

 the pigment sheaths. Similarly, light from b, stimulates two ommatidia at/ ; 

 light from c, two ommatidia at g ; and light from d, one ommatidium at h 

 (from Miiller, 1826 ; by permission of the Cambridge University Library ; 

 by courtesy of Dr. Pirenne and the Pilot Press). 



correspond approximately to an acuity of 1/60 in the human eye. 

 It is important to realize that owing to the isolating effect of the pig- 

 ment mantle, no formed image is produced at the level of the receptor 

 cells ; each of these acts only as a photometer and from the mosaic 

 thus formed by the individual ommatidia the picture of the outside 

 world is synthesized in the central nervous system (van der Horst, 1933). 



