Royal Astronomical Society, B41 



The Astronomer Royal then proceeded to describe some of tht 

 difficulties to which instruments of this class are yet liable, founded 

 partly upon his own observations with Lord Rosse's telescope. 



Upon directing the telescope to an object very near the zenith, it 

 was seen very well defined, or at least with no discoverable fault. 

 It must be remarked that the image of a star never assumes the neat 

 spherical form to which the eye of an observer with a fine refractor 

 is so much accustomed. This arises evidently from the circumstance 

 that (from the great aperture of the mirror) the diffraction image and 

 diffraction rings are invisibly small, and the form of the blurred 

 image is probably determined by the irregular sensibility of the ner- 

 vous membranes of the eye. The same effect exactly is produced 

 by a large refractor when a power is employed too low to exhibit 

 the rings. 



But when the telescope was directed to a star as low as the equator 

 its image was very defective. The defect, however, followed that 

 simple law which the present Master of Trinity College has described 

 by the word astigmatism. When the eyepiece was thrust in, the 

 image of the star was a well-defined straight line, 20 seconds long, 

 in a certain direction ; when the eyepiece was drawn out a certain 

 distance (about half an inch from the former position) the image of 

 the star was a well-defined straight line, 20 seconds long, in a di- 

 rection at right angles to the former. Between these two positions 

 the image was elliptical, or, at the middle position, a circle of 10 

 seconds diameter. The image of Saturn (then without a ring) was, 

 in the two positions above-mentioned, an oval (not an ellipse) whose 

 length was about double its breadth ; or, in the middle position, it 

 was a confused circle, whose diameter was about 30 seconds instead 

 of 20. The position of the astigmatic lines had no distinct relation 

 to the vertical plane ; and this circumstance, as well as the magnitude 

 of the astigmatism, proved that it was not produced by a tilt of the 

 mirror. 



Lord Rosse immediately suggested a probable cause of this defect. 

 The triangular levers which support the mirror are all, to a certain 

 degree, elastic. When the telescope is dropped from the vertical 

 position, the edge of the mirror begins to press the fixed pillars in 

 the fixed frame mentioned under No. II. ; and as the edgewise pres- 

 sure increases faster than the excess of the elastic force of the levers 

 over the pressure on the levers, the edge is firmly locked to the pil- 

 lars by the friction against them. When the telescope is much de- 

 pressed, this friction is perhaps not much less than two tons, or is 

 equal to the greatest strain of six horses, all exerting a force per- 

 pendicular to the face of the mirror at a part intended to sustain no 

 such force, and therefore tending to bend the mirror out of shape. 

 The obvious remedy was to suspend the edge of the mirror in such 

 a manner as to leave it free to play in a direction perpendicular to 

 the face of the mirror ; and for this purpose, first an iron hoop, and 

 secondly a chain, were used, the bearing against the pillars being 

 entirely destroyed. The effect was at first very satisfactory ; defi- 

 nition was made very good ; Saturn's ring was well seen on Sep- 

 tember 2 as a narrow line. But in subsequent observations the effect 



