ABSTRACT
In many mammals, such as the dog, information derived from the sense of smell predominates, whilst in others, such as the mole, both touch and smell are important. Humans, however, gain most of the information about our surroundings from our eyes, so we live in a markedly different perceptual world from most other mammals; for us the eye is our most important sense organ. Complex as it is, the human eye has only limited sensitivity. It responds to that part of the electromagnetic spectrum, between wavelengths of approximately 400–700 nm, which we term light’. If all these different wavelengths are present, the signals are interpreted by the brain as white light, whilst if the range is restricted we have the sensation of colour. Short wavelengths represent violet and blue, whilst progressively longer wavelengths give the impression of green, yellow, orange and red. Throughout the whole range of wavelengths the eye is sensitive to changes in brightness, being most sensitive in the green and declining dramatically at the extreme ends of its range. Other properties of light, such as changes in the phase of the waves and changes in the state of their polarization, are not detected by our eyes. It follows that, if we wish to ensure that our eyes respond to any changes imposed by a microscope specimen on the light which illuminates it, then we must make sure that such changes are presented in the final image as changes in brightness and/or colour.
