large Reflecting Telescopes. V6^ 



vantage. In subsequent numbers of this Journal^ I shall have 

 an opportunity of giving a particular account of the different 

 processes and manipulations which I have employed, so that 

 any person of ordinary mechanical skill who may think it 

 worth while to erect the necessary machinery, will be enabled 

 to obtain with certainty the same results. 



As a general inference from all the facts which have come 

 within my observation, I can have no hesitation in stating, 

 that the reflecting telescope is still susceptible of very great im- 

 provement, — that it has by no means reached the utmost limits 

 of perfection. If we except the defects arising from spherical 

 aberration and the inflection of light, which I think are not 

 irremediable, and are, in my opinion, much overrated in prac- 

 tice, the remaining defects are entirely of a practical nature, 

 and to be overcome by practical means, by numerous and ac- 

 curate experiments, such as a patient consideration of the dif- 

 ficulties to be surmounted must necessarily suggest. 



In order to render the following account intelligible, I will 

 endeavour to put the reader in possession of the difficulties he 

 would have to encounter were he to proceed to construct a 

 large telescope in the common way, and the defects he would 

 probably find in the instrument when finished. He w^ould of 

 course first proceed to cast the metal. As earthen-vessels would 

 not be sufficiently capacious, he would employ either iron ones 

 or a reverberating furnace. If he tried iron vessels, before a large 

 quantity of speculum metal, for instance three or four hundred 

 weight, was raised to a proper heat for casting, he would find 

 that the metal had imbibed some of the iron, andwas injured; or 

 perhaps, if he was less fortunate, and the fire had been a little 

 mismanaged, that the speculum metal had promoted the fusion 

 of the iron, and so passed out through the crucible. The reverbe- 

 ratory furnace would then be resorted to. Much difficulty would 

 occur in combating the continual change of the quality of the 

 metal from the exposure of so large a surface to the action of the 

 flame. However, the metal once cast, the next process would be 

 to anneal it. He would then find that the speculum would fly to 

 pieces before it was cool, unless the alloy made use of was less 

 bright, less white, and in every respect inferior to the best 

 speculum metal. The next process is to grind the speculum. 



