Some reflective properties of the tapetum lucidum of the cat's eye.

1. Observation of the fresh, isolated tapetum of the cat with a low-power microscope showed it to be composed of multilayer reflectors (domains) each being less than 6 mum in diameter. It is concluded that these are subdivisions of the intracellular bundles of rodlets seen in electron micrographs.2. Apparatus was devised for illuminating a single domain with a pencil of white light and observing the colour and direction of the reflected light.3. Those domains which lie near the vitreal surface of the tapetum tend to reflect blue light and deeper ones reflect longer wave-lengths; this is attributed to differences in the layer spacings of the domains.4. Each domain has a set of reflecting planes lying within 15 degrees of the general plane of the tapetal surface. In addition, many domains have surfaces inclined to these at angles greater than 20 degrees which also can reflect visible light.5. The mechanism of reflexion is discussed in relation to the latticelike structure of the domain.6. It is suggested that under some conditions the nature of the tapetum may lead to significant local variations in the spectral sensitivity of the visual system.