FERGUSON'S LECTURES 



LECT, 

 VII. 



v -^s^X. 

 The 



reflecting 

 telescope. 



The great inconvenience attending the management 

 , of long telescopes of this kind, has brought them much 

 into disuse ever since the reflecting telescope was invent- 

 ed. For one of this sort, six feet in length, magnifies 

 as much as one of the other a hundred. It was invented 

 by Sir Isaac Neivton, but has received considerable im- 

 provement since his time ; and i(J now generally con- 

 structed in the following manner, which was first pro- 

 posed by Dr. Gregory. 



amusement ; and in every kind of telescope but the achromatic, they 

 have, with a due degree of perseverance, a fair hope of success. But 

 in attempting to form an achromatic object-glass, however well they 

 may think they have selected their glass, and proportioned the curva- 

 tures of the surfaces, they will be almost certain to find that a single 

 set of lenses, when combined, produce an effect that disappoints their 

 expectations. To succeed perfectly, they must therefore make, from 

 different parcels of glass, a considerable number of lenses, with slight 

 differences of curvature, and those must be selected which will bear 

 the largest aperture and magnifying power. But this would render 

 the undertaking, for the sake of one or two instruments, an Herculean 

 labour, which would not bring to any private individual an adequate 

 recompence ; and which, from the number of imperfect telescopes 

 which are manufactured by those in the most extensive line of business, 

 it would appear that opticians themselves do not fully enter into the 

 spirit of. It has been very generally said and believed, that Dollond 

 made his original experiments, and constructed those excellent three- 

 foot glasses (which at present bear so high a price, and are considered 

 as inimitable), with one single parcel of glass, which accidentally 

 proved superior to any that has since been produced. Nicholson has 

 rendered it extremely probable that this is a vulgar error ; the pro- 

 prietor of the glass-house having assured him, that the original re- 

 ceipts and practice are still followed in the making of optical glass : 

 that the principal opticians always complain of the bad quality of the 

 glass, but never fail to take the whole quantity he makes at their re- 

 quest; and that, when they renew their orders, they always desire it 

 may be exactly the same as the last. It seems therefore reasonable to 

 conclude, that though different parcels of glass, made according to 

 the same process, may differ a little, yet that as good glass for optical 

 uses may be obtained now as formerly, and consequently as good 

 telescopes, if the same great skill and disregard of expense which 

 Dollond evinced, in adapting the curvatures of his lenses to each 

 other and to the glass, were again resort 'd to. 



