10 ON THE CONSTRUCTION OF 



teles^cope shows best with a certain oblique pencil of rays. Probably it Avill be found 

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

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

 destruction oi' many a ^•aluable surface. 



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

 to re-grind or rather re-fine the surface, for though pitch polishing luis 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 w ith 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 fiour emery is reached. After that only washed 

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

 by Ur. (ireen. 



Five ])()unds of the finest sifted flour emery are mixed with an ounce of pulverized 

 gum arable. Knougli watxn* to make the mass like treacle is then addt-d, and the 

 ingredients are thoroughly incorporated by the hand. They are })ut into a deep jar 

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

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

 extremely fine emery in suspension is decanted or drawn ofl' with a si])hon, nearly 

 down to the le\el of the ])recipitated emery at tlu^ l)ottom, and set aside to sid)si(le 

 hi a tall vessi-1. When this has occurred, which will bi' in the laj)S(^ of a few hours, 

 the fluid is to be carefully jjuured back into the first vessel, and the fine de])()sit in 

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

 ci])itate again, emery that has been suspended 30, 10, 3, 1 minutes, and 20, 3, 

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



Th(? (puuitity of the finer emeries consumed in smoothing a 15^ iiicli surface is 

 \-ery trifling — a mass of each as large as two peas suflficing. 



Rouge, or peroxide of iron, is better bought than prepared Ity the amateur. Tt 

 is made by calcining suljiliate of iron and washing the product in water. Tliree 

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

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

 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 PifrJi. 



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 liimg against the side of a house. By 



