40 REFLECTING TELESCOPE. 



The law of its As the pitch is (in our present inquiry) to be considered as 

 giving way. gj^ homogeneous substance, we must suppose, that its resist- 

 ing force, as well as that of the pressure of the mirror on it, 

 are uniformly diffused over the surface of ihe polisher: and, 

 from hence, it may not, perhaps, be easy to conceive, how the 

 surface of the mirror could sustain from it any inequality of 

 resistance and friction. In fact, these would be equal and 

 uniform, in every part, if the pitch were a substance, either 

 of perfect hardness, or perfect fluidity : but it will hereafter 

 appear, that its consistence must not be so hard, as to render 

 it incapable of any change of form : but, on the contrary, so 

 soft, as to yield, in a small degree, to the pressure of the 

 mirror; at the same time, opp^ing a resistance, sufficient to 

 wear down and polish it : and the inquiry is, how that resist- 

 ance is modified. 

 iTr^cedeTuni- Bodies of perfect hardness, such as glass, flints, &c. will not 

 formly, admit a total intimate change of their form, in all their dimen- 



sions, without a dissolution and permanent separation of all 

 the particles composing their masses, (except when they aro 

 brought to a state of fusion by heat). But soft, viscid, semi- 

 fluid bodies, such as lead, pitch, &c. will suffer such change, 

 preserving the cohcision of their particles, yet, at the same 

 tinie, undergoing a general intestine motion of all the par- 

 causes, it becomes necessary, in order to establish the true cause, 

 not only to deduce its existence and effects solely from reasoning 

 on physical principles, but also to obviate other different conjec- 

 tures that might be formed, by stating fully those circumstances 

 that take place in this operation ; and which, indeed, are neces' 

 sary to be clearly understood in judicious practice. Both these, 

 ends cannot be answered, in a disquisition new and intricate, 

 without a minute explanation : and this, I hope, will be received 

 as my apology, for the prolixity of this account, which I would 

 gladly have curtailed, if I knew how to do so, without making 

 it less intelligible or useful to the practical optician. This class of 

 readers will forgive any diffusiveness on a mechanical subject, if 

 the perusal may tend to spare them the greater labour of fruitless 

 experiments ; or afford any hint towards conducting them more 

 judiciously t and as for their use this paper was designed, I have 

 adverted to such various matters as I thought most worthy their 

 attention; and which yet have not been so fully and familiarly 

 explained by others, as they osght to be, for the instruction of 

 au artist. 



