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recedes from the head. It is therefore always more or less fan-shaped. Toward the end it fades away so gradually that it is impossible to say how far the eye can trace it.

Comets differ enormously in brightness, and, notwith standing the splendid aspect which the brighter ones assume, the great majority of these objects are quite invisible to the naked eye. Such are called telescopic comets. There is, however, no broad distinction to be drawn between a telescopic comet and a bright one, there being a regular range of brightness from the faintest of these objects to the most brilliant. Sometimes a telescopic comet has no visible tail; this, however, is the case only when the object is extremely faint. Sometimes, also, the nucleus is almost wholly wanting. In such a case all that can be seen is a small hairy mass, like a very thin cloud, which may be a little brighter in the centre.

From the historical records it would appear that from twenty to thirty comets visible to the naked eye generally appear in the course of a century. But when the telescope was employed in sweeping the heavens it was found that these objects were more numerous than had been supposed. Quite a number are now found every year by diligent observers. Doubtless the number depends very largely on accident, as well as on the skill applied in the search. Sometimes the same comet will be found independently by several observers. The credit is then given to the one who first accurately fixes the position of the comet at a given time, and telegraphs the fact to an observatory.