864 KANSAS ACADEMY OF SCIENCE. 



the glass to polish, but care must be taken not to rub any exposed 

 surface. 



The diagonal or some small glass surface should be silvered before 

 the large mirror to test the purity and efficiency of the silvering mix- 

 tures. The diagonal may be silvered face down in any vessel not too 

 small to receive it. Care should be taken in this not to immerse the 

 mirror until the bubbles have disappeared, and the glass should be 

 held in a slanting position to prevent the lodgment of stray bubbles 

 or floating sediment on the surface. 



COLLIMATION. 



The mounting and collimation of the mirrors is not difficult, with 

 proper caution. An assistant is necessary for this, and indeed very 

 convenient. 



After a tentative fastening of the mirrors, the operator should take 

 his place before the eye tube, at a distance of several inches from it, 

 and, by working the adjustment screws, get the center of the specu- 

 lum, which may be previously marked by a little disk of white paper 

 pasted on the silvered surface of the large glass, to coincide with the 

 center of the diagonal glass, when that mirror is placed so as to show 

 the same length of the supporting wires on each side. A few minutes 

 will usually suffice for this. Care should be taken not to have any 

 undue pressure upon any part of the speculum. A test of this is 

 that the glass should lie loosely in position. Pressure upon any part 

 will result in an annoying flare of a star image, or even a double image. 

 Since this glass lies free in its place, it is better to manipulate the 

 telescope so as to have the large mirror always at the lower part of 

 the tube. 



While these directions may sound formidable to the amateur, they 

 are really simple and easily followed, and the result should be a re- 

 newed telescope, ready for satisfactory work. A good test of accurate 

 collimation is to let an astronomical object travel across the field of 

 view and if the definition is equally good in all positions, the mirrors 

 are properly set. 



