46 MR. BARLOW'S TELESCOPE WITH A FLUID CONCAVE LENS. 



ever, be turned round with comparative ease in about a nainute. The two ends of 

 the rope, as will have been understood, are fixed to the upper curb, crossing 

 each other about 2 feet, viz. double the distance of the guide pulleys where 

 they pass through holes in the curb to the inside, one of them being left so as 

 to allow of taking in the slack of the rope, which is requisite in the beginning : 

 but after being in use a few days this operation is no longer necessary, the 

 rope being every where protected from the weather ; viz. the part round the 

 curb by the eaves of the roof, and the two descending parts by an external 

 casing of wood. As the stand will work from very nearly a horizontal position 

 to a vertical one, it was at first intended that, after the building was completed, 

 the upper curb should be cut away, on one side, the breadth of the shutters, 

 and its place supplied by an iron bolt ; and thus, by having a shutter in the 

 upper flat part of the roof, the instrument might have been brought vertical. 

 This, however, has not been done ; so that at present the limit of observation is 

 between 10° and 65° ; and through this range the instrmnent may be managed 

 with the greatest possible facility by one person. 



Such is the general description of my operations ; and for the rest, I have 

 only to express my hope, that this attempt to introduce a new principle of 

 construction for achromatic telescopes will be examined with candour and im- 

 partiality : that the instrument is so complete and delicate in its action as the 

 most perfect refractors which constitute the chefs-d'oeuvre of opticians, will 

 scarcely be expected. To produce such results requires a great deal of well 

 directed practice, and selections from numerous attempts. I trust, however, 

 I may say that the principle has been shown to be practicable, and that the 

 result is by no means unsatisfactory : and when I state, that, with less than an 

 ounce of the sulphuret of carbon, of the value of three shillings, I have sup- 

 plied, in point of material, the place of the most perfect lens that could be pro- 

 cured of flint glass 8 inches in diameter, it will at least be admitted that the 

 success of the experiment is not altogether uninteresting to the patrons and 

 promoters of astronomical science. 



I will only add, that I should feel no hesitation in undertaking the con- 

 struction of another telescope of double the dimensions of the present. 



