1894.] on the Astronomical Telescope. 313 



being able to increase to any great extent the powers of our telescopes, 

 and with this also naturally arises a question which, judging by the 

 number of queries that reach me about it, seems highly interesting 

 to the general astronomical world. " Will the great telescopes of 

 the future be Eefractors or Reflectors ? " 



Mr. Alvan Clark, whose large refractors in the United States 

 testify to his great skill, declares emphatically for refractors, 

 perhaps naturally so, but his reasons do not appear altogether 

 convincing, and there are others well qualified to judge who give an 

 opposite opinion. It is a question which only the future can decide. 

 Of course, if we all make up our minds that the coming telescope is 

 to be a refractor it will be so, for all our energies will be devoted to 

 its development ; but the same might be said of the reflector, which, 

 I believe, is capable of being greatly improved if attention were 

 directed to it. 



There is one reason that I believe has been overlooked, which 

 explains to some extent why the reflector has not been developed of 

 late years as has the refractor. This matter is not of a scientific, 

 but purely of an economic character, and I should, perhaps, ask 

 pardon fur introducing it into a scientific lecture ; still, it is neces- 

 sary for explanatory purposes. 



Reflectors are, unfortunately for themselves, much less costly 

 than refractors, and I believe that this has much to do with their 

 comparatively neglected condition at present ; this may seem curious, 

 but it is easily explained. An object-glass of 18 inches is worth, 

 say 1000/. ; a mirror of 18 inches is worth, say 100L No one who 

 wanted to have good mounting would object to pay 1000Z. to mount 

 the 10U0Z. object-glass, but there are many who would object to pay 

 the same 1000Z. to mount the 100Z. mirror ; and yet why should it 

 not be equally well mounted ? and if not so, how can it be expected 

 to give as good results as the refractor ? As a matter of fact there 

 are greater difficulties in mounting a reflector than a refractor, and 

 these greater difficulties mean increased cost for an equally good 

 mounting. I believe this simple economic question has much to 

 answer for in bringing the reflector into disrepute with many. It 

 has often been remarked that the reflectors that have been best 

 worked have been constructed and worked by amateurs, the reason, 

 to a great extent, being that this economic point does not then 

 enter so largely into the question. 



There are great difficulties in the mounting of reflectors, more 

 especially when required for use as a photographic telescope, and 

 these difficulties have never yet been satisfactorily solved, but I 

 believe there is nothing unsolvable in them, and that it only wants 

 attention to be drawn to them to ensure a solution. 



Only within the last month Dr. Johnstone Stoney has devised a 

 most ingenious arrangement for supporting the great mirrors of 

 reflecting telescopes on an air support, graduating the pressure 

 according to the angle of inclination of the telescope by an automatic 



