4 THE MODERN REFLECTING TELESCOPE. 



is heavily ribbed on its lower surface, and is connected to its supporting triangle 

 by means of three large leveling screws. The surface of the lai'ge plate was 

 turned and then ground apjiroximately flat; two thicknesses of Brussels carpet are 

 laid upon this, and the glass, with its lower surface previously ground flat, rests 

 upon the innumerable springs formed by the looped threads of the carpet. No 

 better support for a glass during grinding and polishing could be desired. 



Three adjustable iron arcs at the edge of the glass serve for centering tlie 

 latter upon the turntable, and prevent it from slipping laterally. 



The entire turntable, with the heavy frame of wood and metal which supports 

 it, can be turned through 90° about a horizontal axis, thus enabling the optician . 

 to turn the glass quickly from the horizontal position which it occupies during 

 grinding and polishing, to a vertical position for testing. This is shown in Plate ii. 



The tui-ntable is slowly rotated on its vertical axis by means of the large pul- 

 ley below (Plate in). This I'otation is effected by means of belting from the main 

 vertical crank-shaft on the east end of the machine ; this shaft is well shown at the 

 left in Plate iv. At the upper end of this shaft is the large crank, with adjustable 

 throw or stroke, which moves the large and strong main arm to which the grinding 

 and polishing tools are connected, and by means of which they are moved about upon 

 the glass. This I shall always refer to as the main arm. It is a square tube of 

 oak wood, and is strong enough to carry the counterpoising lever shown in Plate iv, 

 and the weight of any of the grinding tools, when fully or partially counterpoised. 

 This main arm also carries the system of pulleys and belts by which the slow rota- 

 tion of the grinding and polishing tools is rigorously controlled ; these, and the 

 manner in which this rotation is effected, are well shown in Plate iv. 



The west end of the main arm consists of a strong steel shaft w^hich slides in 

 a massive bronze swivel-bearing which corresponds to the " elliptical hole in the 

 oak block /) " of Dr. Draper's machine (see his Fig. 25). But this bearing is not 

 stationary as in Draper's machine ; it is not only mounted on a long slide (which I 

 shall refer to throughout this article as the transverse slide), so that it can be 

 slowly moved for several feet across the west end of the machine by means of a 

 long screw, but this bearing and slide are carried upon a secondaiy strong arm, 

 which is moved by a secondary crank at the southwest corner of the machine. 

 Unfortunately there is no phot<)gra[)h which shows this part of the machine as it 

 appears when in use; Plates ii and in show the secondary crank well, but the secon- 

 dary arm is shown swung around with one end I'esting on a bracket on the w"all, in 

 order to have it out of the way. 



The arrangement of the west end of tlie machine is the result of experience 

 with several machines, and is found extremely serviceable and convenient. The 

 long transverse slide on the secondary arm allows the grinding and polishing tools 

 to be placed so as to act on any desired zone of the glass, from the center to the 

 edge; and this setting can be chansjed as desired while the machine is running. 

 The secondary crank, which turns at the same speed as the lai'ge one which drives 

 the main arm, enables the optician to change as desired the width of the (approxi- 

 mately) elliptical stroke or path of the tool with reference to the length of this 



