170 Mr, A. A. Common [May 30, 



mirrors could be used to direct light from any object into a fixed 

 telescope. This idea in a modified form has often been used for 

 special work, one plane mirror being used as we see in the picture 

 on the screen to throw a beam of light into a telescope fixed horizon- 

 tally ; for certain kinds of work this does admirably, but the range 

 is restricted as can be easily seen, and the object rotates in the 

 field of view as the earth goes round. The next step would be to place 

 the telescope pointing parallel to the axis of the earth and send the 

 beam of light into it from the mirror, which could now be carried by 

 the tube so that by simply rotating the tube on its own axis the object 

 would be kept in the field of view. Sir Howard Grubb makes a 

 small telescope on this plan, and some years ago proposed a somewhat 

 similar plan. A sketch of this plan I will show you. You will see, 

 however, that here again the range is restricted, and to use the tele- 

 scope, means would be required to constantly vary the inclination of 

 the small mirror at one-half the rate of inclination of the short tube 

 carrying the object-glass. 



By the use of two plane mirrors, however, the solution of the 

 problem of a rotating telescope tube placed as a polar axis is solved. 

 By having such a telescope with a plane mirror at an angle of 45° to 

 the axis of the telescope in front of the object-glass, we can, by 

 simply rotating the telescope, see every object lying on the equator ; 

 and by adding another similar plane mirror at an angle of 45° to the 

 axis of the telescope, as bent out at right angles by the first plane mirror. 

 and giving the mirror a rotation perpendicular to this axis, we obtain 

 the same power of pointing the telescope as we have in the equatorial. 

 The idea of doing this was published many years ago, but it was left 

 to the skill and perseverance of M. Loewy, of the Paris Observatory, to 

 put it into practical use. He devised, and had made, a telescope on 

 this principle, of 10^ inches aperture, which was completed in 1882. 

 It has proved itself an unqualified success, and many other larger ones 

 are now being made in Paris, including one of 23 inches aperture, 

 now nearly completed, for the Paris Observatory. 



A lantern copy of a drawing of this latter telescope will be thrown 

 on the screen, in order that you may see what manifest advantages 

 exist in this form of telescope. There is but one objection that can 

 be urged — that is, the possible damage to the definition by the 2)lane 

 mirrors ; but this seems, from what I have seen of the wonderful per- 

 fection of the plane mirrors made by the Brothers Henry, to be an 

 unreasonable one — at any rate not an insurmountable one. In every 

 other respect, except perhaps a slight loss of light, this form of tele- 

 scope is so manifestly superior to the ordinary form that it must 

 supersede it in time, not only for general work, but for such work as 

 photography and spectroscopy. 



