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 
basilar 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 
