168 3Ir. A, A. Common [May 30, 



than the inner when it is tested at the centre of curvature. The 

 diagram on the wall, copied roughly from one of the records, I keep 

 of the progress of the work on a mirror during the figuring, shows 

 how this system of measurements enables one to follow closely the 

 whole operation. 



The use of silver on glass as the reflecting surface is as important 

 an improvement in the astronomical telescope as the invention of the 

 achromatic telescope. It gives a permanency to a good figure once 

 obtained that did not exist with the mirrors of speculum metal. To 

 restore the surface of silver to the glass speculum is only a small 

 matter now. How readily this is done may be seen by the practical 

 illustration of the method I will give. I have here two liquids — one 

 a solution of the oxide of silver, and another a reducing agent, the 

 chief material in solution being sugar. I pour the two together in 

 this vessel, the surface of which has been cleaned and kept wet by 

 distilled water, which I shall partly empty, leaving the rest to mix 

 vrith the two solutions ; you will see in the course of about 5 minutes 

 the silver begin to form, eventually covering the whole surface with a 

 brilliant coating that can be polished on the outer surface as bright 

 as that you will see through the glass. 



Eeflecting telescopes have advantages over the refracting telescopes 

 in many ways, but in some respects they are not so good. They give 

 images that are absolutely achromatic, while the other form always 

 has some uncorrected colour. They can be made shorter, and as the 

 light-grasping power is not reduced by the absorption of the glass of 

 which the lenses are made, it is in direct proportion to the surface 

 or area of the mirror. They have not had in many cases the same 

 care bestowed upon either their manufacture or upon their mounting 

 as has been given in nearly every case to the refracting telescope. 

 Speaking generally, the mounting of the reflecting telescope has 

 nearly always been of a very imperfect kind — a matter of great con- 

 sequence, for upon the mounting of the astronomical telescope so 

 much depends. To direct the tube to any object is not difficult, but 

 to keep it steadily moving so that the object remains on the field of 

 view requires that the tube should be carried by an equatorial mount- 

 ing of an efficient character. The first essential of such a mounting 

 is an axis parallel to the axis of rotation of the earth. The tube, being 

 supported on this, will follow any celestial object, such as a star, by 

 simply turning the polar axis in a contrary motion to that of the earth, 

 and at the same rate as the earth rotates on its axis. If we make 

 the telescope to swing in a plane parallel to the polar axis, we can 

 then direct the telescope to any part of the sky, and we have the 

 complete equatorial movement. There are several ways in which 

 this is practically done : we can have a long open-work polar axis 

 supported at top and bottom, and swing the telescope in this, or 

 we can have short strong axes. As examples of the first, I will 

 show you pictures of the mountings designed for Cambridge and 

 Greenwich Observatories some forty years ago by Sir G. Airy, 



