v4 4 % at eee 
224 Miscellanies. 
render the part on which I am about to enter more generally intelligible, 
let me say, that the Newtonian telescope is composed of a large con- 
cave speculum, of a small flat speculum, and of an eye-glass. The 
large concave speculum lies in the closed end of the tube at right angles 
to the tube’s axis. The small flat speculum is placed near the open end 
of the tube in its centre, but at half right angles with the tube; whilst 
the eye-glass (a hole for the purpose being pierced in the tube’s side) 
is fixed opposite the centre of the flat speculum. The rays from the 
object to which the telescope is directed, fall on the large concave 
speculum, are reflected from it into the focus, in which the image of © 
the object is formed ; this image falls on the small flat speculum, and is 
reflected from it to the ees by which it becomes magnified, and 
enters the observer’s eye. But only a part of the light which falls on 
the large concave speculum is reflected on the small speculum ; and 
again, only a part of that which falls from the large speculum on the 
small one is reflected from the latter to the eye-glass. Newton, to avoid 
this loss of light by the second reflection, proposed the substitution of a 
glass prism for the small flat speculum; but from some difficulties 
which have attended its use, it has (perhaps too hastily) been laid aside. 
‘In 1728 Le Maire presented to the Académie des Sciences the plan 
of a reflecting telescope, in which the use of the small flat speculum 
was suppressed ; for by giving the large concave speculum a little incli- 
nation, he threw the image formed in its focus near to one side of the tube, 
where an eye-glass magnifying it, the observer viewed it, his back at 
the time being turned towards the object in the heavens; thus the light 
lost in the Newtonian telescope by the second reflection was saved. ” 
Lord Rosse has determined to give to the great telescope the Le- 
mairan form; but what will then be its power it is not easy to divine ;— 
“‘ what nebule will it resolve into stars; in what nebule will it not find 
stars; how many satellites of Saturn will it show us; how many will 
it indicate as appertaining to Uranus; how many nebul never yet seen 
by mortal eye will it present to us; what spots will it show us on the 
various planets ; will it tell us what causes the variable brightness of 
many of the fixed stars; will it give us any information as to the con- 
stitution of the planetary redial will it exhibit to us any satellites 
encircling them; will it tell us why the satellites of Jupiter, which gen- 
erally pass over Jupiter’s face as discs nearly of white light, sometimes 
traverse it as black patches; willitadd to our knowledge of the physical 
construction of nebulous stars; of that mysterious class of bodies which 
surround some stars, called, for want of a better name, ‘ photospheres 44 
will it show the annular nebula of Lyra merely as a brilliant luminous 
ring; or will it exhibit it as thousands of stars arranged in all the sym- 
metry of an ellipse ; will it enable us to comprehend the hitherto in- 
