12 REPORT — 1851. 



be one of considerable complexity. In an attempt to deduce these expressions at 

 length, it has been found that the expressions become extremely complicated, though 

 it seems difficult to say ■whether they may not still yield to proper treatment : the 

 main question is, whether the expenditure of time and trouble would not be greater 

 than any results likely to be obtained would repay. 



The Earl of Rosse said that, having obser\ed in the London Papers that the Pre- 

 sident of the Association had in his inaugural address conferred upon him the honour 

 of alluding with approbation to the attempts which he had lately made, and with 

 considerable success, to procure plain specula of silver for reflecting telescopes, he 

 thought that perhaps the Section might wish to hear some particulars, and if they 

 could spare a few minutes he would make a short statement on the subject. 



It is well known that about one-third of the light which falls upon the great spe- 

 culum of the Newtonian telescope is lost in the first reflexion, and that nearly one- 

 third of the remainder is lost in the second reflexion. Light being of the greatest 

 importance, especially in the examination of faint objects, in the Herschehan 

 telescope the second reflexion has been dispensed with altogether ; and in the New- 

 tonian telescope attempts have been made, by substituting a prism for the flat mirror, 

 in some degree to reduce the amount lost. In the Herschelian telescope the mirror 

 is inclined, so that the light proceeding in a direction parallel to the axis of the tube 

 is reflected to the centre of the eye-glass fixed to the side of the tube ; there is thus 

 but one reflexion. A consequence, however, of placing the great mirror obliquely 

 is, that though it may be truly parabolic, yet a pencil of light proceeding from a 

 point in an object will not be reflected to a point as it should be, but will be difi^used 

 over a certain space. In a telescope of 3-feet aperture and 27-feet focus, the dia- 

 meter of that space will be more than yi^thof an inch, so that the magnifying power 

 employed cannot be considerable without producing indistinctness. \Vere it pos- 

 sible to work the surface assigned by theorv' for oblique reflexion as accurately as 

 the surface of the paraboloid, we should have the light without the indistinctness ; 

 but that has not yet been accomplished. Where specula are very large, it has been 

 proposed to place the eye-glass in the axis of the tube, and it has been contended 

 that the light interrupted by the head and shoulders of the observer would be less 

 than the light lost by the second reflexion. This is no doubt true, and various ways 

 have been suggested of placing the observer so that the light interrupted should be 

 a minimum, the temperature of the air in the tube remaining at the same time un- 

 disturbed. In such a construction, however, there would be great diflrraction, and 

 that appears to me to be an insurmountable objection. The rectangular prism w£is 

 proposed by Newton as a substitute for the plain speculum ; and with a prism of 

 i>A inches aperture by Mertz, the saving of light is considerable. Mertz informed 

 me that a somewhat larger prism might be made, but that there would be consider- 

 able delay : he did not however hold out any hopes of being enabled to make one 

 large enough for our 3-feet speculum. It is evident that as the size of the prism is 

 increased, the amount of light lost in passing through the glass will be greater, and 

 a point will at length be cirrived at, sooner or later, according as the glass is more or 

 less transparent, when the light lost in prismatic reflexion will be as great as in re- 

 flexion from a speculum of metal. It occurred to me that a small prism might be 

 substituted for a large one by placing it near the focus, and that practically the in- 

 convenience of a small field might be obviated to a certain extent by employing a 

 plain speculum and eye-piece in the ordinary way for general work, to be turned 

 aside by a suitable contrivance when the prism was to be made use of. An eye- 

 piece, of course of an unusual construction, would be required, cuid it does not seem 

 practicable to make such an eye-piece with two lenses achromatic : the four-glass 

 eve-piece would be so, but some light would be sacrificed. How far the want of 

 achromatism would interfere with real work I have not proceeded far enough to be 

 able to say. Achromatic prismatic refraction has been proposed by Sir David 

 Brewster as a substitute for the plain speculum. It has not, as far as I am aware 

 of, been tried, but the great size of the prism which would be required appears to 

 me to be a serious objection. 



Under these circumstances, it is obvious that where there was a reasonable pro- 



