ASTRONOMICAL TELESCOPES — PEASE 205 



Warner and Swascy for use in their large refractors. Years of 

 use have i3roved its superiority to any others we have yet tried. 



Perhaps the final factor which enables us to build the large instru- 

 ments we have to-day is the silver-on-glass mirror. The fact that 

 we can take a piece of glass such as the 100-inch disk and cut the 

 curve in ils surface once for all is a matter of very great importance. 

 The glass forms the permanent curve, but it is actually the very thin 

 silver film that is the opaque reflector. Unlike the speculum mirrors, 

 the reflecting surface is very easily replaced without the least injury 

 to the curve or the mirror disk itself. The silvering of the 100-inch 

 disk, for example, is accomplished by removing the mirror in its 

 cell from the telescope and lowering it by an elevator into the silver- 

 ing room below. A metal band is wrapped around the edge of the 

 mirror, forming a vessel or bowl. The old silver film is removed with 

 acid, the mirror is washed, and then a fresh film precipitated from 

 a solution poured into the bowl. Waste solutions are drained off 

 through the spout into a large tank below. A thorough burnishing 

 with chamois skin and rouge removes from the film any trace of 

 whiteness which may form during the silvering. 



Let us now study the optical parts of a telescope in order to under- 

 stand them a little better. We have two methods of forming optical 

 images. In one the light passes through a transparent lens, while 

 in the other the material is opaque and light is reflected from its 

 surface. We take advantage of the fact that light travels more 

 slowly in transparent solids than in air and that by placing differ- 

 ent thicknesses in the beam we can retard one portion with respect 

 to another. To form an image, all the rays in a parallel beam must 

 reach the focus at the same time. As illustrated in Figure 1, the 

 outermost ray naturally has to travel a greater distance, so very little 

 glass is put in its path. A little further in, the beam is not bent so 

 much ; hence the angle of the prism is made less, and at the center the 

 glass has parallel surfaces. Notice that as we approach the center we 

 make the glass thicker and thicker. If we were skillful enough, we 

 could take a very large number of pieces of glass and place them 

 together; but we can do still better — we shall use only one piece of 

 glass and make the surfaces such as to imitate the action of all the 

 individual prisms. For our ordinary lenses the curve we use is a 

 sphere, as it is most easily made with our grinding tools. 



In the case of a mirror (fig. 1) we want the parallel light to reach 

 (lie focus by equivalent paths, so we take a number of small mirrors 

 and place them so that all the total distances arc the same. Notice 

 that the outer ones are brought forward a little and tipped differ- 

 ently with respect to one another and to the inner one which is 

 squared on. If we should replace the small blocks of glass by a 

 single piece and grind a surface to imitate the action of the separate 



