Royal Irish Academy. 587 



The certainty of the process is such, that the solid mirror of thirty- 

 six inches aperture, after being scratched all over its surface with 

 coarse putty, was, in Dr. Robinson's presence, perfectly polished in 

 about six hours, and was placed in its tube for examination without 

 any previous trial as to quality. 



Lord Oxmantown has preferred the Newtonian to the Herschelian 

 form, and, in Dr. Robinson's opinion, with good reason. In the latter, 

 the inclination of the great mirror to the incident rays must deform the 

 image*, and it is now known, that even with faint objects sharp de- 

 finition is of high importance. It should, in fact, be a segment of a 

 paraboloid, exterior to the axis ; and though a theorem of Sir Wil- 

 liam Hamilton (Trans. Roy. Irish Acad., vol. xv. p. 97) might seem 

 to indicate mechanical means of approximating to the figure, yet Dr. 

 Robinson fears there would be greater difficulty in applying them than 

 in enlarging the aperture of the Newtonian, so as to make up for the 

 loss of light. Another serious objection is, that in the Herschelian 

 the observer's position at the mouth of the tube must cause currents of 

 heated air, which will materially interfere with sharpness of definition. 



As to the loss of light by the second reflexion, Dr. Robinson thinks 

 it has been much overrated, and expresses a wish that a careful set of 

 experiments were made on reflexion by plane specula at various inci- 

 dences, on prisms of total reflexion, and the achromatic prism, pro- 

 posed as a substitute by Sir David Brewster. 



As to the rest of the instrument, it may suffice to say, that it bears 

 a general resemblance to that of Ramage, but that the tube, gallery, 

 and vertical axis of the stand are counterpoised, so that one man can 

 easily work it, notwithstanding its enormous bulk. The specula, 

 when not in use, are preserved from moisture or acid vapours, by con- 

 necting their boxes with chambers containing quicklime, which is oc- 

 casionally renewed. This arrangement (which also occurred to Dr. 

 Robinson, and has been for several years applied by him to the Armagh 

 reflector,) appears to be very effective in preserving the polish/ 



In trying the performance of the telescope, Dr. Robinson had the 

 advantage of the assistance of one of the most celebrated of British 

 astronomers, Sir James South ; but they were unfortunate in respect 

 to weather, as the air was unsteady in almost every instance ; the 

 moonlight was also powerful on most of the nights when they were 

 using it. After midnight, too (when large reflectors act best), the 

 sky, in general, became overcast. The time was from October 29th 

 to November 8th. 



Both specula, the divided and the solid, seem exactly parabolic, 

 there being no sensible difference in the focal adjustment of the eye- 

 piece with the whole aperture of thirty-six inches, or one of twelve ; 

 in the former case there is more flutter, but apparently no difference 

 in definition, and the eyepiece comes to its place of adjustment very 

 sharply. 



* Any one who has a Newtonian telescope can verify this, by inclining a 

 little the great mirror, so however as not to pass the edge of the plane mir- 

 ror by the pencil. In Lord Oxmantown's instrument, an inclination of 11' 

 sensibly injures it; were it Herschelian, the inclination must be 3° 11'. 



2Q2 



