REFLECTINS TELESCOPE. 39. 



by each : the cause of either being to me then unknown ; 

 and disappointment or success appearing to depend on mere 

 accident, and not on the degree of pains and accuracy used in 

 the process. 



At length I began to suspect, that these variation<;, in the The methods 

 event of the process, (which will be hereafter accounted for,) Edwards vary 

 arose from some property, not adverted to, in the pitch that from the pro- 

 covered the polishing tool ; which material has been generally ^f^^^^ °^ '^* 

 used for this purpose, of communicating a proper figure, as 

 well as a high polish, to the mirror, since it was first recom- 

 mended by Sir Isaac Newton ; being commonly spread on 

 the polisher, to about the thickness of a crown-piece, and 

 then covered with the polishing powder : (the manner of 

 doing which I Suppose the reader to be acquainted with, as 

 also with what has been made public on the subject, by Messrs. 

 Hadley, Mudge, Edwards, &c. ;) and I was confirmed in ray 

 suspicion, from the following reasons, after I had found them 

 approved by many repeated and diversified experiments. 



Pitch is a soft unelastic substance, which, as such, will The pitch, 

 suffer a permanent change of form, when it is made to sustain yieidlng,^"^ ' * 

 a degree of pressure sufficient to communicate an intestine 

 motion to its particles: and this property directs us to con- 

 sider, what may be the effect of the pressure of the mirror on 

 it, when spread on the polisher, as to the figure it may then 

 gradually acquire, daring the operation of polishing, and the 

 resistance and friction it will oppose to the mirror : for, by 

 reason of the tenacity of its substance, it will resist a certain 

 degree of pressure, without change of its form, but will yield 

 to a greater pressure. Bat it is by its resistance the mirror 

 is worn down and polished ; if, therefore, that resistance be 

 not uniform and equal, on the whole surface of the polisher, 

 neither will the abrasion of the mirror be equal in every part; 

 the consequence of which must be, that both will degenerate 

 from an uniform curvature, i. .e. from a spherical figure ; the 

 mirror from unequal friction, and the polisher from its mobiHty, 

 by which it will adapt to the successive alterations produced . 

 in the figure of the mirror ; their mutual action and reaction 

 inducing a change in both.* . 



* This change, however, being so little, as to be imperceptible 

 \3^ the senses, and, in the imagination, referable to various othef 

 P4 



