Note 97 — Chapter III

There is reason for believing that a large proportion of animal eyes see much as ours do when in a normal state. Colour blindness is frequent in Man and occurs between red and green, yet a bull distinguishes the two like a healthy, human being. He is allured by the sight of a green field, and lashes himself into fury when a red rag is waved before him.

The eyes of insects are very far removed in structure from ours. A butterfly's compound eye contains 17,000 tubes, that of the Mordella beetle 25,000. Their perception of colours appears vivid and distinct. They resemble birds, reptiles, and other creatures in choosing for their lairs and resting-places objects coloured like themselves. It is not difficult to mount one of these compound eyes, so as to look through it by aid of a lens placed in focus. Leeuwenhoeck looked through the eye of a dragon fly (made up of 12,544 tubes), "and viewed the steeple of a church which was 299 feet high, and 750 feet from the place where he stood. He could plainly see the steeple, though not apparently larger than the point of a fine needle. He also viewed a house in the same manner, and could discern the front, distinguish the doors and windows, and perceive whether they were open or shut." See Insect Miscellanies, p. 129.