175 



the-second ej'e-glass, than that of the inner zone; since 

 it is necessary to withdraw that focus, by putting back 

 the little mirror: and the contrary is evident, in the lat- 

 ter case. The former could happen, only hy the focus 

 of the extreme rays, of each single pencil, being too far 

 from the great speculum, (i. e. fiom its being hypei'bo- 

 Jical,) and too near to the little one; or from the latter 

 being deficient in curvature, near its edges; and thus 

 throwing the focus of the rays, that fall there, too far 

 fiom it, and too near to the last eye-glass. The second 

 effect could arise only from a figure of the mirrors, the 

 reverse of this. In the Newtonian telescope, there can 

 be no doubt, where the defect of curvature is, because it 

 has but one concave mirror. 



• When it has been thus determined what the defect is, 

 means must be employed to correct it; and it may be 

 expected, that, unless some certain mode, of effecting a 

 different curvatuie of the great mirror, from that of the 

 little one, is discovered, and skilfully practiced, there will 

 be but few good telescopes, of the Gregorian form, con- 

 structed. For, if both mirrors be polished, in the same 

 manner and method, it is likel3', that the defects in their 

 figure, and the species of their curvature, will be the 

 same in both. Whereas, it has been shewn, that all these 

 ought to be directly contrary in One, from what they are 

 in the other; referring to the parabola and ellipse, as the 

 standard degrees of curvature. 



z 2 Now, 



