« A SILVERED GLASS TELESCOPE. 19 



examination must always be regarded as illuminated by an oblique light coming 

 from a source on the side opposite to that from which the screen advances, coming 

 for instance from the left hand side, in the above description. 



In practice, the diaphragms commonly used for a 151 inch mirror have been as 

 small as the light from the unsilvered surface would allow. A six inch aperture at 

 the centre, a ring an inch wide round the edge, and a two inch zone midway between 

 the two. 



e. Machines. 



In the. beginning of this section the difficulties into which I feU with Lord Rosse's 

 machine were stated. These caused it at the time to be abandoned. A machine 

 based on the same idea as Mr. LasseU's beautiful apparatus was next constructed. 

 It varied, however, in this, that the hypocycloidal curve was described partly by the 

 rotation of the mirror, and partly by the motions of the polisher — the axes of the 

 spindles carrying the two being capable either of coincidence or lateral separation 

 to a moderate extent. A great deal of time and labor was expended in grinding 

 and polishing numerous mirrors with it, but still the difficulty that had been so 

 annoying in the former machine persisted. Frequently, in fact generally, from six 

 to eight zones of unequal focal length were visible, although on some occasions 

 when the mirror was hyperbolic, the number was reduced to two. At first it was 

 supposed that the fault lay with the polishing, the pitch accumulating irregularly 

 from being of improper softness, for it was found to be particularly prone to heap 

 up at the centre. But after I had introduced a method of fine grinding with elu- 

 triated hone powder, which enabled the glass to reflect light before the pitch 

 polishing, it became evident that the zones were connected with the mode of 

 motion of the mechanism. Many changes were made in the speed of its various 

 elements, and a contrivance to control the irregular motion of the polisher intro- 

 duced, but a really fine and uniform parabolic surface was never obtained, the very 

 best showing when finished zones of different focal lengths. Although it cannot 

 be said that I have tried this machine thoroughly, for Mr. LasseU has produced 

 specula of exquisite defining power with it, and must have avoided these imperfec- 

 tions to a great extent, yet the evident necessity of complicating the movement' 

 considerably, to avoid the polishing in rings, led me to adopt an entirely difiierent 

 construction, which Avas used until quite recently. Although it has now been 

 replaced by another machine, which is still better in principle, and gives fine results 

 much more quickly, yet as it produced one parabolic surface that bore a power of 

 more than 1000, and as it serves to introduce the process of grinding, it is worthy 

 of description. The action of machines for grinding and polishing has been 

 thoroughly examined in my workshop, no less than seven different ones having 

 been made at various times. 



* Messrs. De La Rue and Nasmyth, who used one of Mr. LasseU's machines, as I have since 

 learned, met with the same trouble, and were led to make two additions to the mechanism : 1, to 

 control the rotation of the polisher rigorously ; and 2, to give the whole speculum a lateral motion, 

 by which the intersecting points of the curves described by the polisher were regularly changed in 

 distance from the centre of the mirror. Mr. Lassell had previously, however, introduced a contrivance 

 for this latter purpose himself. 



