THE MODERN REFLECTING TELESCOPE. 3 



which is nearly ou a level with the ground outside, is of cement and is heavily 

 painted. The w^alls are of brick, are about two feet thick, and are covered with 

 two layers of heavy ceiling paper arranged so as to give two tight one-inch air- 

 spaces for constant-teiiiperatiire purposes. All j<Mnts of the paper are lapped and 

 are nailed down with strips of wood. The ceiling of the room is heavily varnished. 



The large I'oom is divided into three rooms connected by large doors; these 

 doors are so ari'anged that the entire length of the large I'oom and of a wide hall 

 opening from it, making an a[>artnieut 165 feet long, can be utilized for testing. 

 The east and middle rooms of the three are used for grinding and polishing. The 

 large windows of these rooms are fitted with storm sash on the inside ; these are built 

 in permanently and are made air-tight by means of ceiling paper. The west room 

 contains the motor which supplies power to the grinding and polishing machines in 

 the inner rooms ; power is transmitted by a long shaft which runs the entire length 

 of the rooms ; this shaft is built in air-tight (to prevent dust) beneath the long 

 work-bench which runs along one side of the rooms. 



With these arrangements temperature, moisture, and freedom from dust can be 

 controlled in the grinding and polishing rooms with all necessary refinement. In other 

 respects, however, three great improvements could be made in planning an ideal op- 

 tical shop ; two of these relate to the comfort and health of the optician. First, the 

 rooms should be arranged so that direct sunlight could be admitted to them during 

 all parts of the optical work in which this would not be injurious to the work itself. 

 Second, provision should be made for supplying to the rooms an abundance of 

 fresh air, of a definite temperature, and washed free from dust. Third, for constant- 

 temperature purposes, walls and partitions covered with a heavy layer of asbestos 

 plaster (commercially termed Asbestic) would be preferable, on account of the 

 superiority of the insulating and fire-proofing qualities of this material, to those of 

 ceiling papei' with air-spaces. 



CHAPTER III. 

 GRINDING AND POLISHING MACHINES. 



The grinding and polishing machines used by the writer are somewhat similar 

 in principle to Dr. Draper's machine, shown in Fig. 25 of his book, but are more 

 elaborate. I shall describe here the machine used in making the 5-foot mirror, 

 both because it embodies most of the essential featui-es of a grinding and polishing 

 machine, and also because it is the only one of my machines of which I have a 

 series of photographs for illustration. A good idea of this machine may be gained 

 from the views of it shown in Plates ii, in, iv, and vi. 



The massive turntable ui)on which the glass rests consists of a vertical shaft 

 or axis five inches in diameter, carrying at its upper end a very heavy triangular 

 casting, upon which, in tui'n, is supported the circular plate upon which the glass 

 lies. This plate is of cast-iron, weighs 1,800 pounds, is 61 inches in diameter, 



