10 ON THE CONSTRUCTION OF 



telescope shows best with a certain oblique pencil of rays. Probably it will be found 

 that this circumstance is by no means peculiar to this telescope." This very valuable 

 observation has lain buried for eighty-two years, and ignorance of it lias led to the 

 destruction of many a valuable surface. 



As regards the method of combating this tendency, it is as a general rule best 

 to re-grind or rather re-fine the surface, for though pitch polishing has occasionally 

 corrected it in a few minutes, it will not always do so. I have polished a surface 

 for thirteen and a half hours, examining it frequently, without changing the obliquity 

 in the slightest degree. 



Glass, then, is a substance prone to change by heat and compression, and requiring 

 to be handled with the utmost caution. 



b. Emery and Rouge. 



In order to excavate the concave depression in a piece of glass, emery as coarse 

 as the head of a pin has been commonly used. This cuts rapidly, and is succeeded 

 by finer grained varieties, till flour emery is -reached. After that only washed 

 emeries should be permitted. They are made by an elutriating process invented 

 by Dr. Green. 



Five pounds of the finest sifted flour emery are mixed with an ounce of pulveri/ed 

 gum arabic. Enough water to make the mass like treacle is then added, and the 

 ingredients are thoroughly incorporated by the hand. They arc put into a deep jar 

 containing a gallon of water. After being stirred the fluid is allowed to come to 

 rest, and the surface is skimmed. At the end of an hour the liquid containing 

 extremely fine emery in suspension is decanted or drawn off with a siphon, nearly 

 down to the level of the precipitated emery at the bottom, and set aside to subside 

 in a tall vessel. When this has occurred, which will be in the lapse of a few hours, 

 the fluid is to be carefully poured back into the first vessel, and the fine deposit in 

 the second put into a stoppered bottle. In the same way by stirring up the pre- 

 cipitate again, emery that has been suspended 30, 10, 3, 1 minutes, and 20, 3, 

 seconds is to be secured and preserved in wide-mouthed vessels. 



The quantity of the fineiyemeries consumed in smoothing a 15| inch surface is 

 very trifling a mass of each as large as two peas sufficing. 



Rouge, or peroxide of iron, is better bought than prepared by the amateur. It 

 is made by calcining sulphate of iron and washing the product in water. Three 

 kinds are usually found in commerce : a very coarse variety containing the largest 

 percentage of the cutting black oxide of iron, which will scratch glass like quart/ ; 

 a very fine variety which can hardly polish glass, but is suitable for silver films ; 

 and one intermediate. Trial of several boxes is the best method of procuring that 

 which is desired. 



c. Tools of Iron, Lead, and Pitch. 



In making a mirror, one of the first steps is to describe upon two stout sheets of 

 brass or iron, arcs of a circle with a radius equal to twice the desired focal length, 

 and to secure, by filing and grinding them together, a concave and convex gauge. 

 When the radius bar is very long, it may be hung against the side of a house. By 



