,50 REFLECTING TELESCOPfe: 



all which shew that their cohesion, when a fluid does Intervene, 

 is not caused by the pressure of the atmosphere. 



Agreeably to this, the sagacious Newton directs, that, 

 towards the end of the operation, no more moisture should be 

 applied to the polisher, than what it will contract, from the 

 operator's breathing on it. Indeed, a person, who has formed 

 a just conception of his genius and intense application of mind 

 and considered the hints and precepts he has given in this work, 

 can hardly doubt, that he could, and, perhaps, would, have 

 furnished a theory of the rules and method of this whole process; 

 had he not imagined it would, at that time, be regarded as 

 a matter of too little importance, to deserve so rninute an ex- 

 planation, wiiich must be necessarily prolix, and seem un- 

 worthy of him, who was occupied in more sublime speculations, 

 Havd pitch From this it follows, that, when the pitch is of unyielding 



will not give a hardness, it will not, in any mode of polishing, communicata 

 goo gure, ^^ ^^^ mirror the desired shape, if the dust worn from the 

 mirror, does not alter the shape of the polisher. And, as 

 this seems not likely to happen, so I was not surprised, that 

 my efforts, to effect the desired figuration of the mirror, by 

 using very hard and refractory pitch, failed of success. 



And there is this inconvenience, moreover, in the use of 

 such pitch, viz. that it makes so great resistance to the sinking 

 and bedding of the polishing powder in it that the particles of 

 the powder, however fine it may be, will, on any fresh appli- 

 cation of it, or when any grains of it are accidentally dislodged 

 from the pitch, roll about loose on the polisher, and scratch 

 the face of the mirror, so as to destroy the polish before given ; 

 -~noT will it ^^^^^ making any fresh application of the powder inadmissible, 

 receive the po- unless the pitch were to be softened by heating it, which would 

 ingpow r. j^^^gjj.Qy jj^g former figure, and render the operation uncertain 

 and tedious. It was to allow the polishing powder to fix itself, 

 without rolling loose on the polisher, and to suffer all its parti- 

 cles, however different in size, to sink in it, so as to form an 

 even surface, that Sir Isaac Newton, in his sagacity, employed 

 a coat of pitch on the polisher, as a soft substance, that would 

 yield to the powder, when impressed on it by the mirror, and 

 not afford such resistance, as to make it fret the face of the 

 metal; and also as a substance endued with another property 

 equally necessary, that of being perfectly iinelastic. For no 

 elastic substance will ever communicate an exquisite polish to 



