VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 175 



them are so exceedingly small, compared to the latter, and the number of de- 

 grees employed so very small, that the inconvenience of a spherical figure is not 

 so much perceived. Accordingly we observe in the generality of reflectors 

 (whose specula, unless by accident, are always spherical) that the only true rays 

 which form the distinct image arise from the middle of the metal : and unless 

 the defect be remedied bv a considerable aperture, which destroys much light, the 

 false reflection from the inside of the metal produces a greyish kind of haziness, 

 which is never seen in Mr. Short's, or indeed in any good telescopes. 



Supposing that the 2 foci of the different parts of the metal perfectly coincide, 

 and that, by the union of them when the apertures are removed, the telescope 

 shows the objects very sharp and distinct, you are not however even then to 

 conclude that the instrument is not capable of further improvement ; for you will 

 perceive a sensible difference in the sharpness of the image, under different po- 

 sitions of the great speculum with respect to the little one, by turning round the 

 great metal in its cell, and opposing different parts of it to different parts of the 

 little metal, correcting by this means the error of one by the other. This at- 

 tempt should be persevered in for some time, turning round the great speculum 

 about t'-j- at a time, and carefully observing the most distinct situation each time 

 the eye-piece is screwed on : when, by trying and turning the great metal all 

 round, the distinctest position is discovered, the upper part of the metal should 

 be marked with a black stroke, that it may always be lodged in the cell in the 

 same position. This is the method Mr. Short always used; and the caution is 

 of so much consequence, that he thought it necessary to mention it very par- 

 ticularly in his printed directions for the use of the instrument. Mr. Short 

 also frequently corrected the errors of the great by the little metal in another 

 way. If the great speculum did not answer quite well in the telescope, he cured 

 that defect sometimes by trying the effect of several metals successively, by this 

 means correcting the errors of one by the other ; for in several of his telescopes 

 which have passed through my hands, when the sizes and powers have been the 

 same, I have found that the great metals, though very distinct in their proper 

 telescopes, yet have, when taken out and changed from one to the other, spoiled 

 both telescopes, rendering them exceedingly indistinct, which could arise from 

 no other circumstance. For this reason I suppose it was, that he kept ready 

 finished, a great many large metals of the same focal length, so that, when he 

 wanted to mount a telescope, he might from a great choice, be able to combine 

 those metals which suited each other best. I am strongly inclined to believe this 

 was the case, not only from the above observation, but because he showed me 

 himself a box of finished metals, in which I am sure there were a dozen and a 

 half of the same focal length. 



There is another circumstance, and a material one, which must not be omitted; 



