large Reflecting Telescopes. 143 



The compound one when adjusted was somewhat superior. 

 The polish of both was very good, but the surface of course 

 was not so. The solid speculum was about as good as the 

 average of similar metals which I have seen. When these 

 metals have been ground and polished by machinery, further 

 trials shall be made with them. 



After the experiments which I have just described, I deter- 

 mined to defer for the present expending any further labour 

 upon the eighteen inch compound speculum, and resolved to 

 endeavour previously to discover some certain method of giv- 

 ing specula a more accurate surface. I was confirmed in that 

 determination from an apprehension that the castings were not 

 as perfect as they should have been. All my workmen were 

 trained in my own laboratory without the assistance of any 

 professional person, and none of them had previously seen 

 any process in the mechanic arts ; and I was not myself then 

 acquainted with the precautions necessary to insure the pro- 

 duction of an alloy of zinc and copper in the due proportions. 

 There was^ therefore, a great probability that the castings 

 were defective. 



The polishing apparatus described in No. xviii. of this 

 Journal was completed about that time. It has since under- 

 gone some alterations. The different motions are now ob- 

 tained by cog wheels and leather bands. Several other minor 

 alterations have also been made, both in the apparatus and in 

 the manner of conducting the whole process, which have pro- 

 duced the most material improvement. The results obtained 

 by machinery are very nearly uniform. Where a uniform 

 combination of motions produces a defect, that defect will uni- 

 formly recur, and may, therefore, with great facility, be traced 

 to its course and corrected. Such has repeatedly been the 

 case. The same specula have been repolished a great number 

 of times, and the performance of the machine has improved 

 faster than I could have anticipated. 



The practical optician will rarely give you the slightest in- 

 timation of the process of working specula which he finds the 

 most successful, nor is it perhaps to be expected. It is there- 

 fore impossible to describe with certainty his mode of proceed- 

 ing ; but I believe the practice is to work the speculum till it 



