Rods can respond to light but also to pressure. Voltage can set off most receptors. Works very well with touch receptors not as well with more diffuse receptors such as pain. Touch Receptors can adapt to a stimulus - put an ear ring on pretty soon you don't know it is there.
Light comes through the pupil.
When examining something close, the cillary muscle are contacted relaxing the zonular fibers which allows the lens to become more spherical. Optic disk - blind spot where optic nerves comes through retina no rods or cones. Each photopigment is bound to a chomophobe molecule called retinal, a Vit. Most domestic animals have limited color vision. Colors are a blend of these 3 pigment being stimulated - like mixing colors.