telescope, meets, in a more convenient and satisfactory manner than any other previous device for the purpose, all the observing requirements for work at the upper end of the tube. Mechanism for Silvering In order to renew the silver coating on the upper surface of the mirror, which is necessary about three times a year, mechanism has to be provided for handling the mirror and its cell, the lower section of the telescope tube, with ease and safety. The mirror weighs over 2 tons and the cell 4 tons, so 6 tons have to be removed from and replaced on the telescope tube. This is effected by means of the silvering car, which can be seen at the left of Fig. 2, a massive framework of structural steel rolling on four flanged wheels on flush tracks in the observing floor. With the tube turned to a vertical position, the car is rolled from its normal position at the east side of the dome directly under the tube and a motor-operated screw-jack surmounted by a triangular rocking arm can be brought up against the bottom of the cell. On removing the attaching bolts, cell and mirror can be lowered and rolled on the car to the east out of the way. The removal of 6 tons from one side throws the telescope out of balance and so the outboard end of the declination axis is supported by a counterweighted strut run up through the floor and the upper end of the tube tied to rings at the top of the dome. With a band of paraffined paper tied around the edge of the mirror and a plug in the central hole, the silvering solution can be poured on and evenly flowed over the surface by rocking on a steel ball at the top of the jack-screw. When the silvering is complete, cell and mirror are replaced in the reverse order and the car rolled back out of the way. The operation takes about a day and is performed with perfect safety and ease. [Illustration: Fig. 2.--TELESCOPE IN AVERAGE OBSERVING POSITION] SECTION 3.--MECHANICAL PARTS OF TELESCOPE Introduction It is useful, before describing the mechanical parts or mounting of the telescope, to explain the difference between the two kinds of telescope, refracting and reflecting, employed in astronomical work. The refracting telescope is the most familiar type as the ordinary spyglass or draw-tube telescope and the field or opera glass are all refracting telescopes. The refracting telescope is so called because the light from the distant object is refracted through a lens at the outer end of the tube and forms an image of the object at the inner end, just as a camera forms an image on the ground glass or film, and this image is viewed and magnified by the eyepiece or ocular. The reflecting telescope on the other hand has the upper or outer end of the tube open and the light from the distant object is reflected (hence the name) from a concave mirror at the lower end of the tube, forming the image of the object at the top, where it can be viewed and magnified by the ocular as in the refractor.