46 
MR. BARLOW’S TELESCOPE WITH A FLUID CONCAVE LENS. 
ever, be turned round with comparative ease in about a minute. 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 instrument 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. 
