<prompt> As per Young-Helmholtz theory colour revealing are of___ types2345  <response>: Ans. b 3 Ref. Parsons, Disease of Eye, 19th131; Khurana, Ophthalmology, 3rd250; Ganong, Physiology, 21166 Normal color vision is trichromatic, the basis of Young-Helmholtz theory of colour vision.YOUNG-HELMHOLTZ THEORY Cones are photoreceptors tightly packed in fovea centralis where there are no rods and other layers of retina very thin. The central part of fovea consists of cones and their nuclei largely and all other retinal layers are absent. The appreciation of colors is function of cones and occurs in photopic vision. Sensation of any colour is determined by the relative frequency of impulses for cone system. Normal color vision is trichromatic, the basis of Young-Helmholtz theory of colour vision. Other theories of color vision are- Hering or Hurvich and James Law trichromatic signal from cones fed into subsequent neural stage and exhibit spectral opponent and spectral non-opponent property, and- Edwin-Land wavelength theory. 