CASSKUKAIN BEFLEOTOR WITH OOBBEOTBD FIKI.D. 



I would add a few remarks upon the problems presented by the construction of 

 such a telescojM', or at. any rate, i>t' its optical parts. It requires the production of a 

 great mirror and three lenses which shall be in due relation to one another. None 

 of the sizes or curves go outside what has already been made ; and whenever a 

 refractor is made, three of the surfaces must be turned out in agreement with the 

 fourth. Hence there is no new difficulty in making and the problem is essentially a 

 question of testing. The testing must be optical and not mechanical, for the former 

 far outruns the latter in delicacy it is said ten times. And because there are so 

 ninny surfaces it would 1 essential to test them independently of one another. In 

 the lenses, four out of the six surfaces are concave and spherical and can be tested 



AXIS 



Fig. 3. Koverser and corrector. Scale 1 : 3. 



with reflected light. The great mirror is neither a sphere nor a paraboloid, but its 

 radius of curvature for different zones can be laid down, and each zone tested for 

 agreement with this, just as in making a paraboloid. There remain then two convex 

 surfaces, and the question of figuring the lens-surfaces to allow for inequalities of 

 refractive index within the glass. These are matters for the skill of the maker and 

 it would seem a not unreasonably difficult task. 



I add a plan of the whole instrument and, upon a larger scale, of the reverser and 

 corrector, and also the final specification, collected from pp. GO, G2, but making the 

 unit 1 inch. For comparison the field of a Newtonian of the same aperture and focal 

 length is added. It may be recalled that the displacement of the centre of the comatic 

 circle is twice the comatic radius. For an uncorrected Cassegraiu the field would be 

 very much the same as for a Newtonian of the same aperture but of focal length equal 

 to that of the great mirror, except in respect to curvature and distortion, see p. 41. 



I would express my acknowledgments to Mr. R. W. WKIGLEY who helped me to 

 perform many of the calculations. 



VOL. CCXIII. A. K 



