570 THE SENSES AND SENSORY ORGANS. 



antero-posterior diameter of the eyeball, the distinctness of 

 vision if this hypothesis were correct would diminish with the 

 size of the eye. Now, the evidence goes to prove that birds 

 see quite as distinctly as we do ; a Hawk with an eyeball having 

 an antero-posterior diameter of 5 or 6 mm. should not see 

 objects as distinct which subtend an angle less than 3' or 4', 

 whilst in man the estimated angle is less than i' (59" is usually 

 given). It is therefore probable that each retinal element in 

 such birds produces a complex sensation, and the same maybe 

 and probably is, true in insects. So that although on the above 

 hypothesis an insect such as the Blow-fly appears to have only 

 ^V of the acuity of vision which man possesses, if it were com- 

 pared with a bird, it is possible that \, or even J, would be a 

 nearer estimate ; and it must be remembered that Dragon-flies 

 and Hymenoptera have vision which is many times sharper 

 than that of the Blow-fly when calculated on the same 

 method. My own estimate of the vision of the Dragon-flies 

 (sEschna) and Wasps (Vcspa) is that they have seven or eight 

 times the acuity of vision possessed by the Fly, and this would 

 equal that of the Hawk or Swallow a conclusion which is, I 

 think, borne out by the experience of those who have watched 

 these insects in a state of nature. 



The Illumination of the Retinal Image. Every microscopist is 

 aware that the higher the magnifying power and the smaller the 

 front lens of the objective the greater is the need to increase the 

 illumination. It will be manifest that the smallness of the corneal 

 lenses necessitates a brilliant light, if, as I have maintained, a 

 real image is formed at the inner extremity of the great rods. 

 As a matter of fact most insects require a brilliant light, and 

 are only active when the sun is shining. Many insects, as the 

 Noctuids and Sphingidae, are, however, remarkable exceptions. 

 In these the large size of the great rods is conspicuous, and 

 I h;ive observed the same thing in all nocturnal insects, and 

 especially in the Tipulidai and Gnats. 



Many nocturnal insects are remarkable for the large size of 

 tin,- corneal facets, and the great reduction in the number of 

 the oinmatea in the compound eye. There is a marked con- 



