108 THE COCKROACH : 



House-fly as it avoids the hand, of a Bee flying from flower to 

 flower, or of a Dragon-fly in pursuit of its prey. The sight of 

 such Insects as these must range over several feet at least, and 

 within this field they must be supposed to distinguish small 

 objects with rapidity and certainty. How can we suppose that 

 an eye without retinal screen, or accommodation for distance, is 

 compatible with sight so keen and discriminating ? The answer 

 is neither ready nor complete, but our o\vn eyesight shows how 

 much may be accomplished by means of instruments far from 

 optically perfect. According to Aubert, objects, to be perceived 

 as distinct by the human eye, must have an angular distance of 

 from 50" to 70", corresponding to several retinal rods. Our 

 vision is therefore mosaic too, and the retinal rods which can 

 be simultaneously affected comprise only a fraction of those 

 contained within the not very extensive area of the effective 

 retina. Still we are not conscious of any break in the con- 

 tinuity of the field of vision. The incessant and involuntary 

 movements of the eyeball, and the appreciable duration of the 

 light-stimulus partly explain the continuity of the image 

 received upon a discontinuous organ. Even more important is 

 the action of the judgment and imagination, which complete 

 the blanks in the sensorial picture, and translate the shorthand 

 of the retina into a full-length description. That much of what 

 we see is seen by the mind only is attested by the inadequate 

 impression made upon us by a sudden glimpse of unfamiliar 

 objects. We need time and reflection to interpret the hints 

 flashed upon our eyes, and without time and reflection we see 

 nothing in its true relations. The Insect-eye may be far from 

 optical perfection, and yet, as it ranges over known objects, the 

 Insect-mind, trained to interpret colour, and varying bright- 

 ness, and parallax, may gain minute and accurate information. 

 Grant that the compound eye is imperfect, and even rude, if 

 regarded as a camera ; this is not its true character. It is 

 intended to receive and interpret flashing signals ; it is an 

 optical telegraph. 



Plateau* has recently submitted the seeing powers of a 

 number of different Insects to actual experiment. The two 

 windows of a room five metres square were darkened. An 



* Bull, de 1'Acacl. Roy. de Belgique, 1885. 



