176 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



it is this : for the very same reason that the pitch should not be too hard or soft, 

 the work will not proceed well in the heat of summer, or the cold of winter : in 

 the latter, it may be possible to remedy the defect by having the room warmed 

 with a stove ; and in the summer, the other inconvenience may perhaps be 

 avoided by using a harder kind of pitch ; but I much doubt in either case 

 whether the work will go on so kindly : I have my-olf always wrought in spring 

 and autumn. The process of polishing, and indeed grinding on the hones, will 

 not go on so well if it be not continued uninterruptedly from beginning to end ; 

 for if tlie work of either kind be left but for a quarter of an hour, and you then 

 return to it again, it will be some time before the tool and metal can get into a 

 kindly way of working ; and till tliey do, you are hurting what was done before. 



Magnifying very minute objects, and particularly reading at a distance, have 

 been generally considered as the surest tests of the goodness of a telescope; 

 and indeed when the page is placed at a great distance, so that the letters sub- 

 tend but a very small angle at tlie eye, if then they appear with great precision 

 and sharpness, it is most probable that the instrument is a good one. But yet we 

 are sometimes apt to be deceived by this method ; nor is it always possible to 

 determine on the different merits of 2 instruments of equal power, by this mode 

 of examination ; for when the letters are removed to the utmost extent of the 

 powers of the 1 instruments, the eye is apt to be prejudiced by the imagination. 

 If 2 or 3 words can be here and there made out, all the rest are guessed at by 

 the sense ; insomuch that an observer, zealous for the honour of his instrument, 

 is very apt to deceive himself in spite of his intentions. The surer test is by 

 figures, where you can procure no aid from this sort of deception. In order to 

 examine my reflecting telescopes, I made, on a piece of copper and on a black 

 ground, 6 lines consisting of about J 2 pieces of gold figures, and each line of 

 figures differing in magnitude, from the smallest that could be ilistinctly made, 

 to those of about -^ of an inch long ; besides, the figures in the several lines 

 were differently disposed, and the sum of each line also differed. It is evident 

 that by this method all guess is precluded ; and that, of 2 instruments, of the 

 same powers, that which can inake out the least order of figures, whii.h will be 

 known by llie sum, is the best telescope. Such a plate I caused to be fixed up 

 for experiments against the top of a steeple, about 300 yards north of my house ; 

 and it will serve to give some idea of the distinctness with which very small 

 figures could be made out at that distance, by saying, that in a clear state of the 

 air, and with the sun behind me, with a telescope of ]8 inches focal length. I 

 nave seen the legs of a small fly, and the shadows of them, with great precision 

 and exactness. : ji .• 



I cannot conclude without indulging myself in an observation on the amazing 

 sagacity of Sir Isaac Newton, in every subject on which he thought fit to 



