ABSTRACT
The ideal criterion is that passing from one class to the next, the eye senses the same difference of visual stimuli. However, the human eye can reliably judge this difference only over small angular distances and for objects of approximately identical color, so that the classification is not coherent over large arcs on the celestial sphere. In this chapter, the authors examine point-like, self-luminous objects (the stars), with only brief considerations of extended luminous objects such as nebulae or galaxies, and of illuminated objects, either point-like, such as asteroids, or extended, such as comets and planets. Often, telescopes employ refracting elements, either as primary optical elements or inside their instrumentation. The human eye, actually its retina, is a very good detector of light, even though it is a nonlinear device. A very useful concept is that of the black body, of such importance in the development of all physics that the people assume it well known to the reader.
