26 O N T n E C N S T R U C T I O X A N I) USE OF 



By this system of local polisliing the difficulties of heat, distrihution of polishing 

 ])o\vders, irregular contact of the rosin, Szc. that render the attainment of a fine 

 tigure so uncertain usually, entirely (lisai)pear. A sjiherical surface is produced as 

 above described, and iifterwards by moving q towards the edg(\ and at tlie same 

 time increasing the stroke, it is con\ crted into a paraboloid. I'lie tieecy a])pearance 

 spoken of on a former page is not perc(nved, and the surface is good almost up to 

 the extreme edge. 



(4.) Ete-Pieces, Plane Mirrors and Test Objects. 



The telescope is fiu'uislu^d with se\eral eye-pieces of various construction, giving 

 magnifying powers from 7-> to TiOO, or if it were desired even higher. For the 

 medium powers yOO and 600 KauLsden, or rather positive eye-pieces have becin 

 adopted. They ditfer, however, from the usual form in being achromatic, that is, 

 ea(;h plano-convex is composed of a flint and ciown, arranged according to formulas 

 calcidated by LittroAW In this way a large flat field and absence of color are 

 secured, and the fine images yielded by the mirror are not injured. For the higher 

 powers, single achromatic lenses are used, and for the highest of all a Ross 

 microscope. 



AMtli theses means it has been found that the parabolic surfaces yielded by tlie 

 processes before described, will define test objects exc(>llently. Of close doubk; 

 stars they will separate such as y" Andromedie, and show the colors of the compo- 

 nents. In the case of unequal stars which seem to be moi'c severe tests, they can 

 show the close companion of Sirius — discovered by Mr. Alvan Clark's magnificent 

 refractor — the sixtli component of (/' Orionis, and a nudtitude of other difficult 

 objects. 



As an example of liglit collecting ])ower, iJebillisima between ;■ and 5 Lyra^ is 

 found to be quintui)l(\ as first noticed by Mr. liassell. In the isl inch specula of 

 Herschel, it was only recorded as doul)]e. and, according to :Vdmiral Smyth, Lord 

 Rosse tlid not notice the fourth and fiftli compomuits. Jupiter's moons show w^ith 

 beautiful disks, and their dift'ereiu'c- in diameter is very marked. As for tlu^ body 

 of that planet, it is literally covered with Ixlts up to the poles. The bright and 

 dark .spots on Venus, and the fading illumination of her inncn' edge, and its irregu- 

 larities are perceived even when the air is far from tranquil. Stars are often seen 

 as disks, and without any wings or tails, ludess indeed the mirror should be wrongly 

 placed, so that the best diameter for siq)port is not in the perpendicidar plane, pass- 

 ing through the axis of the tube. 



It has been found that no ach antage other than the decrease of atmospheric 

 infiuence on the image, results from cutting down the aperture of tliese mirrors by 

 diaphragms, while the disadvantage of reducing the sejjarating power, is perceived 

 at the same time. Faint objects can be better seen Avith tlie whole surface than 

 with a reduced aperture, and this though apparently a property common to all 

 reflectors and object glasses is not so in reality. A defective edge will often cause 

 the wdiole field to be filled with a pale milky light, which will extinguish the fainter 

 stars. Good definition is just as important for faint as for close objects. 



The properties of these mirrors have been best shown by the excellence of tlic 



