340 Mr. mudge on the Conjlruci'wn 
metal ihould be marked with a black ftroke, in order 
that it may always be lodged in the cell in the fame por- 
tion. This is the method Mr. short always ufed; and the 
■ caution is of fo much confequence, that he thought it 
necelfary to mention it very particularly in his printed 
directions for the ufe of the inftrument. 
And farther, Mr. short frequently corrected the 
errors of the great by the little metal in another way. If 
the great fpeculum did not anfwer quite well in the tele- 
fcope, he cured that defeCt fometimes by trying the effeCt 
of feveral metals fucceffively, by this means correcting 
the errors of one by the other; for in feveral of his tele- 
fcopes which have puffed through my hands, when 
the fizes and powers have been the fame, I have found 
that the great metals, though very diftinCt in their pro- 
per telefcopes, yet have, when taken out and changed 
from one to the other, fpoiled both telefcopes, render- 
ing them exceedingly indiftinCt, which could arife from 
no other circumftance. For this reafon I fuppofe it 
was, that he kept, ready finifhed, a great many large 
metals of the fame focal length, fo that, when he wanted 
to mount a telefcope, he might from a great choice, 
be able to combine thofe metals which fuited each 
other beft. I am ftrongly inclined to believe this was the 
cafe, not only from the above obfervation, but becaufe 
r he 
