138 Lord Ox man town on the construction of 



which, though laborious, does not require much exactness, 

 and lastly, to polish it, which every one knows is attended with 

 very great difficulty. Making a probable estimate of the suc- 

 cess likely to be obtained, after a great number of abortive at- 

 tempts, a metal would be completed, having a tinge of yellow 

 deeper in proportion to its size, with perhaps a defective po- 

 lish, and certainly a figure by no means perfect ; such a metal 

 would not bear any considerable power with tolerable distinct- 

 ness. What I have just stated is the result of experience. 

 That I have not overrated the difficulties and defects, will ap- 

 pear evident to any one who is conversant with the late Sir 

 W. HerschePs writings. Since Sir W. HerschePs time, no 

 improvement that I am aware of has been made in any part 

 of the process of making the specula of telescopes ; none of 

 the difficulties which he stated as existing have since been sur- 

 mounted ; and none of the defects which his skill had not re- 

 moved, have since yielded to the dexterity and perseverance 

 of others. 



From the accounts which we have of Sir W. HerschePs la- 

 bours, it appears, that, in proportion as he increased the size 

 of his specula, he was obliged to debase the quality of the 

 metal made use of. Dr Pearson, in his Practical Asirwiomy, 

 states, that the proportion of tin to copper used for the metal 

 of the twenty foot telescope was 7.75 to 20, — an alloy certainly 

 extremely low. He also states that the metal of the forty foot 

 telescope was still lower, and was composed of blocks of an al- 

 loy purchased at a warehouse in London. The weight of the 

 large metal for the forty foot telescope was 1 050 pounds, the 

 diameter four feet eight inches, the thickness at the edge two 

 inches, and in the middle one inch and a quarter. It is diffi- 

 cult to conceive how a metal of such weight, so great a dia- 

 meter, and so little thickness, could retain even a tolerable fi- 

 gure in the different positions of the telescope. 



It is also well known, that Sir William Herschel, at the 

 commencement of his career, polished 400 specula of differ- 

 ent dimensions ; content if he could procure one tolerably good 

 one out of a great number. Such were the difficulties that 

 he had to encounter ; and I am not aware that anything has 

 been pubhshed since that time, tending materially to diminish 



