38 



and therefore Mr. Troughton's preference of three readings to two 

 is fully confirmed. There seems no great advantage to be expected 

 from encreasing the number beyond this, and the difficulty of using 

 and keeping them in order is considerable. 



On the whole I think we may conclude that the construction of our 

 circles is not as defective as some suppose ; they correct their errors, 

 and the only thing which is wanting to make them as perfect as the 

 ; essential properties of matter allow is some effectual check on the sta- 

 sbility of the microscopes. It is impossible to depend on any fixture of 

 them however firm, and we want some means either of eliminating 

 their deviations from our result, or of measuring and allowing for them. 

 -The observations made at Greenwich wilhjixed telescopes, where 

 all the resources of art and science were employed, shew strikingly 

 how impossible it is to fix any thing; and indeed in all things con- 

 nected with instruments, we are not warranted to suppose ought 

 that is susceptible of proof. This is not the least important lesson 

 which we derive from the labours of our predecessors in science. 



T. R. ROBINSON. 



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