<P> Although all vertebrates have this blind spot, cephalopod eyes, which are only superficially similar, do not . In them, the optic nerve approaches the receptors from behind, so it does not create a break in the retina . </P> <P> The first documented observation of the phenomenon was in the 1660s by Edme Mariotte in France . At the time it was generally thought that the point at which the optic nerve entered the eye should actually be the most sensitive portion of the retina; however, Mariotte's discovery disproved this theory . </P> <Table> <Tr> <Th_colspan="5"> Demonstration of the blind spot </Th> </Tr> <Tr> <Td_colspan="5"> </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td_colspan="5"> </Td> </Tr> <Tr> <Td_colspan="5"> Instructions: Close one eye and focus the other on the appropriate letter (R for right or L for left). Place your eye a distance from the screen approximately equal to 3 × the distance between the R and the L. Move your eye towards or away from the screen until you notice the other letter disappear . For example, close your right eye, look at the "L" with your left eye, and the "R" will disappear . </Td> </Tr> </Table> <Tr> <Th_colspan="5"> Demonstration of the blind spot </Th> </Tr>

Consequences if light rays fall on the blind spot