THE MODERN REFLECTING TELESCOPE. 47 



With the present great improvements in the mateiials and methods of machine 

 construction there is no longer anj- excuse for unstable and inconvenient mountings 

 for reflectors. The focal length of modern reflectois intended for photography is 

 short; the ratio of aperture to focal length generally used in such instruments will 

 probably be not greater than as 1 to 4, nor less than as 1 to 6 ; with such 

 i-atios the mounting can be made extremely compact and rigid. By the addition of 

 a small convex mirror the equivalent focal length can be increased from three and 

 one-half to five times, and fine definition retained ; when this is done the actual 

 length of the tube is less than when the telescope is used at the primary focus. 



The reflecting telescope defines well only at or near the optical axis; hence the 

 mirrors must I'emain in perfect adjustment with reference to each other and to the 

 eyepiece or photographic plate, in all positions of the telescope which can occur in 

 use. Not only must the mirror supports be such as to define the position of the 

 miri'ors rigorously alwa3's, as described in the piecediug chaptei', but the short 

 tube must be excessively strong and rigid so that no sensible flexure can occur. 

 This is especially necessary when the telescope is used as a Cassegrain, orasartx^^ ,• 

 for when these forms are employed it is only when the axes of the pai'aboloid and 

 hyperboloid coincide that tine definition can be secured. When the necessity of 

 these conditions is fully realized by makers and users of reflectoi-s, a marked ad- 

 vance in the usefulness of reflecting telescopes will result. It was the lack of such 

 rigidity and of such permanence of adjustments, fully as much as the lack of means 

 of rigorously testing the optical surfaces, which made the old Cassegrain reflectors, 

 including the great Melbourne instrument, such lamentable failui'es. I consider the 

 failure of the Melbourne reflector to have been one of the greatest calamities in the 

 history of instrumental astronomy; for by destroying confidence in the usefulness 

 of great reflecting telescopes, it has hindered the development of this type of 

 instrument, so wonderfully efficient in photographic and specti'oscopic work, for 

 nearly a third of a century. 



When the telescope is to be used for photography, either direct or spectro- 

 scopic, it is indispensable that the mounting be so designed that reversal is not 

 necessary when passing the meridian ; for it is frequently necessary to expose for 

 six or eight hours without reversal, on faint objects; and the best })art of such an 

 exposure is that in which the celestial object is near the meridian. Several forms 

 of reflector mounting have been devised in which reversal is not necessary ; the well- 

 known English closed-fork mounting is one of them. 



In designing the proposed mounting of the 5-foot reflector of the Yerkes 

 Observatory, of twenty-five feet focal length, the writer has adopted the form in 

 which a short open fork is used at the upper end of the polar axis. The tube 

 hangs between the arms of this fork, being carried on two massive trunnions; the 

 heavy lower end of the tube is so short that it can swing through, between the 

 arras of the fork, for motion in declination. 



The fork mounting presents sevei'al marked advantages with respect to com- 

 pactness and stability, as well as convenience and economy, over all forms which 

 are modifications of the German equatorial mounting, in which the tube is carried 



