VISION. 347 



great exactness to the length of the tul)e attached to it; so 

 that an image of an external ohject is formed precisely at 

 the point where the retina is placed to receive it* 



Little is known of the respective functions of these two 

 kinds of eyes, the simple and the compound, both of which 

 are generally possessed by the higher orders of winged in- 

 sects. From the circumstance that the compound eyes are 

 not developed before the insect acquires the power of flight, 

 it has been inferred that they are more particularly adapted 

 to the vision of distant objects; but it must be confessed that 

 the experiments made on this subject have not, hitherto, led 

 to any conclusive results. Duges found, in his trials, that 

 after the stemmata had been covered, vision remained appa- 

 rently as perfect as before, while, on the other hand, when 

 insects were deprived of the use of the compound eyes, and 

 saw only with the stemmata, they seemed to be capable of 

 distinguishing nothing but the mere presence or absence of 

 light. Others have reported, that if the stemmata be co- 

 vered with an opaque varnish, the insect loses the power of 

 guiding its flight, and strikes against walls or other obsta- 

 cles: whereas, if the compound eyes be covered while the 

 stemmata remain free, the insect generally flies away, rising 

 perpendicularly in the air, and continuing its vertical ascent 

 as long as it can be followed by the observer. If all tho 

 eyes of an insect be covered, it will seldom make any at- 

 tempt whatsoever to fly. 



The eyes of insects, whether simple or compound, are 

 immoveably fixed in their situations; but the compound eyes 

 of the higher orders of the class of Crustacea, arc placed at 

 the ends of moveable pedicles, so as to admit of being turned 

 at pleasure towards the objects to be viewed.f This, how- 



* This interesting fact was communicated to me by Captain Kater, who, 

 together with Mr. Children, assisted Dr. Wollaston in this examination. 



f Latreille describes a species of Crab, found on the shores of the Medi- 

 terranean, having its eyes supported on a long jointed tube, consisting of 

 two articulations, which enables the animal to move them in various direc- 

 tions, like the arms of a telegraph. 



