2 THE jrODEKN REFLECTING TELESCOPE. 



Company, for example. The diagonal plane mirror of a Newtonian, and the conv^ex 

 mirror of a Cassegrain reflector, should always be made of the best optical glass, 

 since the expense for these is comparatively slight. 



The writer has used many disks made at the celebrated glass-works of St. 

 Gobain, near Paris, of sizes from 8 inches in diameter and IJ^ inches thick, to 

 the great one shown in the plates accompanying this article, which is 5 feet in 

 diameter and 8 inches thick, and which weighs a ton. All of these disks are 

 beautifully free from bubbles and large striae, and are fairly well annealed, con- 

 sidering their great thickness. It is a most encoui'aging fact that the quality of 

 the 5-foot disk is not infeiior in any I'espect to that of disks of 8, 12, 20, 24, and 

 30 inches diameter which I have used. The makei'S of the 5-foot disk have re- 

 cently expressed their readiness to undertake for us a 10-foot disk, one foot thick, 

 which they think could now be made as perfect in all respects as the 5-foot disk. 

 In ordering these disks it is always specified that great care be given to thorough 

 stirring and thorough annealing. I have no doul)t that in tlie case of ver}^ large 

 and thick disks the makers could be prevailed upon to give even greater care to 

 these points than is now given. 



A very important point is in regard to the best thickness of optical miri'ors. 

 As a result of experience in making and using many mirrors of 24 and 30 inches 

 diameter, in which the thickness of the several disks varies from one-twelfth to 

 one-sixth of the diameter, I have no douVjt that the thicker disks are always prefer- 

 able, provided that they are as homogeneous and well-annealed as the thinner ones. 

 The thinner mirrors suffer much greater teraporaiy change of curvature from the 

 very slight heat generated during the process of polishing; and they are undoubt- 

 edly more liable to suffer temporary disturbance of figure from changes of tem- 

 perature when in use in the telescope. In the cases of the large paraboloidal mirror 

 of a reflecting telescope, and the lai-ge plane miiTor of a coelostat or heliostat, which 

 should always be supported at the back to prevent flexure, the thickness should not 

 be less than one-eighth or one-seventh of the diameter; in the writer's opinion the 

 latter ratio leaves nothing to be desired. In the cases of the small diagonal plane 

 mirror and the small convex mirror, which cannot easily be supported at the back, 

 the thickness should be not less than one-sixth of the diameter. 



All mirrors should be polished (not figured) and silvered on the back as well 

 as on the face, in order that both sides shall be similarly affected by temperature 

 changes when in use in the telescope ; for the same reason the method of supporting 

 the large mirror at the back, in its cell, should be such that the back is as fully 

 exposed to the air as possible. 



CHAPTER II. 



THE OPTICAL LABORATORY OF THE YERKES OBSERVATORY. 



A LARGE, well-lighted room, 70 feet long by 20 feet wide, in the north 

 basement of the Observator}-, was designed for the optical laboratory. The floor. 



