26 ON THE CONSTRUCTION AND USE OF 



By this system of local polishing the difficulties of heat, distribution of polishing 

 powders, irregular contact pf the rosin, &c. that render the attainment of a fine 

 figure so uncertain usually, entirely disappear. A spherical surface is produced as 

 above described, and afterwards by moving q towards the edge, and at the same 

 time increasing the stroke, it is converted into a paraboloid. The fleecy appearance 

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

 the extreme edge. 



(4.) EYE-PIECES, PLANE MIRRORS AND TEST OBJECTS. 



The telescope is furnished with several eye-pieces of various construction, giving 

 magnifying powers from 75 to 1200, or if it were desired even higher. For the 

 medium powers 300 and 600 Ramsden, or rather positive eye-pieces have been 

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

 each plano-convex is composed of a flint and crown, arranged according to formulas 

 calculated by Littrow. 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. 



With these means it has been found that the parabolic surfaces yielded by the 

 processes before described, will define test objects excellently. Of close double 

 stars they will separate such as y 2 Andromeda?, and show the colors of the compo- 

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

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

 refractor the sixth component of 6 1 Orionis, and a multitude of other difficult 

 objects. 



As an example of light collecting power, Debillisima between e and 5 Lyra? is 

 found to be quintuple, as first noticed by Mr. Lassell. In the 18| inch specula of 

 Herschel, it was only recorded as double, and, according to Admiral Smyth, Lord 

 Rosse did not notice the fourth and fifth components. Jupiter's moons show with 

 beautiful disks, and their difference in diameter is very marked. As for the body 

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

 dark spots on Venus, and the fading illumination of her inner 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, unless indeed the mirror should be wrongly 

 placed, so that the best diameter for support is not in the perpendicular plane, pass- 

 ing through the axis of the tube. 



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

 influence on the image, results from cutting down the aperture of these mirrors by 

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

 at the same time. Faint objects can be better seen with the 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 whole 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 the 



