566 THE SENSES AND SENSORY ORGANS. 



by this method, and so learns something of the nature of sur- 

 rounding objects [252, p. 137]. 



Accommodation. That accommodation really occurs in the 

 simple eyes of Spiders is indicated by the scintillations of the 

 luminous reflex when viewed with the micro-ophthalmoscope. 

 The great elasticity of the great rods and rhabdomes and 

 the remarkable muscle which regulates the tension of the 

 basiiar membrane (PL XXXVIII., Fig. 2, m c), render it ex- 

 tremely probable that the ommatea of the compound eye are 

 capable of being adjusted for distinct vision. Exner's theory 

 of super-position reverses the conditions of accommodation, 

 and would require a shortening of the great rods for near 

 vision, and, as Exner remarks, no mechanism is present by 

 which this can be accomplished. The distinct mechanism for 

 the lengthening of the rods is only another argument against 

 an untenable theory. 



It is true that the plane in which the subcorneal image lies 

 varies so little, even for very near objects, that accommodation 

 appears unnecessary, under the dioptric theory ; but it is 

 probable that variations in the tension of the rods themselves 

 occur during life which need a readjustment of the dioptric 

 apparatus. Changes in the blood pressure or the tracheal 

 pressure probably affect the length and tension of the great 

 rods, and it is conceivable that insects accommodate not for 

 nearness, but to counteract other disturbing influences. The 

 dioptron is supplied with blood and air from the general head 

 cavity ; the circulating fluid enters it and leaves it by sinuses 

 round the edge of the membrana basilaris, which have a 

 valvular arrangement in Dragon-flies : and it may be that the 

 contraction of the tensor muscle of this membrane affects its 

 tension long after the contraction has passed away, as such 

 contractions must necessarily alter the conditions under which 

 air and blood pass into and out of the dioptron. The stretching 

 of the great rods would undoubtedly alter their foci, and when 

 once adjusted it may be a long time before readjustment is 

 necessary. 



Extent of the Visual Field. If the extreme angular divergence 



