Meanwhile, Sir Isaac Newton, considering, wrongly as the sequel showed, that 'the case of the refractor was desperate,' set about the attempt to find out whether the reflection of light by means of suitably-shaped mirrors might not afford a substitute for the refractor. In this attempt he was successful, and in 1671 presented to the Royal Society the first specimen, constructed by his own hands, of that form of reflecting telescope which has since borne his name. The principle of the Newtonian reflector will be easily grasped from Fig. 5. The rays of light from the object under inspection enter the open mouth of the instrument, and passing down the tube are converged by the concave mirror AA towards a focus, before reaching which they are intercepted by the small flat mirror BB, placed at an angle of 45 degrees to the axis of the tube, and are by it reflected into the eye-piece E which is placed at the side of the instrument. In this construction, therefore, the observer actually looks in a direction at right angles to that of the object which he is viewing, a condition which seems strange to the uninitiated, but which presents no difficulties in practice, and is found to have several advantages, chief among them the fact that there is no breaking of one's neck in the attempt to observe objects near the zenith, the line of vision being always horizontal, no matter what may be the altitude of the object under inspection. Other forms of reflector have been devised, and go by the names of the Gregorian, the Cassegrain, and the Herschelian; but the Newtonian has proved itself the superior, and has practically driven its rivals out of the field, though the Cassegrain form has been revived in a few instances of late years, and is particularly suited to certain forms of research. [Illustration: FIG. 5.--PRINCIPLE OF NEWTONIAN REFLECTOR.] [Illustration: FIG. 6.--LORD ROSSE'S TELESCOPE.]