234 On the Catoptrical and Dioptrical 



h is *, docs not derogate a tittle from the probability of tire 

 fact in question- For, if this mirror existed, it is probable 

 that it was the only one of its- kind, and that no other mean 

 had then been found of viewing distant objects distinctly. 

 ft must therefore have been considered as a great wonder in 

 those times, and must have filled with astonishment those 

 who saw its feffects. Even though its effects had not been 

 greater than those of a small telescope, it could not fail to 

 be regarded as a prodi<ry. Hence it is natural to think that 

 those effects were exaggerated beyond all probability, and 

 even possibility, as commonly happens to rare and admira- 

 ble machines and inventions f. If we abstract then, from 

 the accounts of the mirror of Ptolemy, the evident exagge- 

 rations of ignorance, nothing will remain, but that at some 

 distance, provided nothing was interposed between the ob- 

 jects and the mirror, those objects were seen more distinctly 

 than with the naked eye ; and that with the mirror many 

 objects were seen, which, because of their distance, were 

 imperceptible without it. Here is nothing but what is both 

 possible and probable. 



56. " Let us next inquire whether the knowledge which 

 the antients had of dioptrics and catoptrics, and especially of 



* It is certain, however, that, under some circumstances, objects may 

 be seen at a greater distance than is 'generally supposed. For example : 

 it is said that the Isle of jV^hu is clearly visible fiom the summit of Ben 

 ' Lomond, in Dumbartonshire, which*cannot be less than a direct distance 

 of 1 20 miles. Mr. Glas, in his History of the Canary Islands, affirms, 

 that the Peak of TenerifTe is visible izo miles in approaching it, and 150 

 in leaving it. The difference, no doubt, arises from the difficulty a man 

 has to distinguish from a cloud the first appearance of a mountain he is 

 approaching, and the ease with which in leaving the mountain he can 

 keep it in view till it gradually disappear. Brydone, if 1 mistake not, 

 -says that, frvjm the summit of Etna,' mountains may be distinguished at 

 the distance of even 200 mile*. Whether in this instance any mention 

 is made of glasses I do not iccollect. But the most extraordinary fact of 

 this kind L have met with, if it be a fact, is to be found in the Encycl. 

 Biitan. art. London, § 22, where we are told that the illumination of the 

 atmosphere bv the great fire at London in 1666, is said to have been vi- 

 sible at Jedburgh, in Scotland. — Translator. 



\ Witness the strange stories propagated about the barometer possessed 

 by David Gregory, of Kinartiie. See Mutton's Diet. art. Gregory; or the 

 Suppl. to the Encycl. Brit, same article. Here I cannot help expressing 

 mv satisfaction at finding that, in dissenting from Sir Isaac Newton, which 

 I did with a trembling pen, respecting the effects of Gregory s warlike 

 engine, I have the learned editor of this Lst excellent work on my side. 

 I need scarcely add, that the same opinion extends to Mr. Gillespie's new 

 invented revolving battery ; which, as a powerful, cheap, and compen- 

 dious instrument of defence, attracts more and more the attention of offi- 

 cers of distinction both naval and military. See my first Letter, towards 

 the end. — Translator. 



the 



