SENSE OF SIGHT. 213 



could only see two colours, black and white, and he could 

 never distinguish the cherries on a tree from the leaves. A 

 tailor at Plymouth could only see yellow and blue. On one 

 occasion he repaired a Hack silk garment with crimson, and 

 on another, he patched the elbow of a blue coat with a piece 

 of red cloth. M. Nicoll tells us of an officer in the British 

 navy, who purchased a blue uniform coat and waistcoat, 

 with red breeches to match. -The cause of this defect is 



y 



believed to be a malformation, or deficiency of that portion 

 of the brain which takes cognizance of colours^ 



36. The lowest order of animals have no organs of 

 vision, or if so, they have never been detected. Many in- 

 sects have two kinds of eyes, one kind on each side of the 

 head, and the other on the top, in a row, or in the form of a 

 triangle. The spider .has generally eight of these eyes on 

 the top of the head. [What are celled the compound eyes 

 are placed on the side of the head, and in the wasp and 

 dragon-fly they cover a large part of it. These eyes are 

 formed of a large number of separate cones or cylinders, 

 closely packed together, each being a distinct eye, and hex- 

 agonal, or six-sided in shape, like the cells of a honey-combl 

 The ant has about 50 ; the beetle 3,000 ; the silk-worm moth 

 upwards of 6,000 ; the dragon-fly 12,000 ; and some in- 

 sects as many as 20,000. 



37. It is now ascertained, that each eye forming these 

 compound eyes of insects, consist of a distinct tube, furnished 

 with every thing necessary for complete vision. The object 

 of this wonderful arrangement seems to be to compensate for 

 want of motion, by the number of eyes, as the insect thus 

 supplied, has an eye pointed towards the object in whatever 

 direction it may appear, and may therefore be truly called 

 circumspect. If we examine the wasp and dragon-fly, we 

 shall find these compound eyes to cover a large portion of 

 each side of the head. By examining with the microscope, 

 we find each of these conical tubes covered with a cornea, 

 and containing a crystalline lens, and aqueous and vitreous 



