1886.] on Telescopic Objectives and Mirrors. 423 



squares in any zone while retaining the same distance from centre 

 to centre of squares, we increase at first the pressure per unit of area 

 on the pitch squares in that zone, and consequently the subsidence will 

 be greater, and the action will not be so tight or severe on that zone. 

 I know of no substance but pitch and a few of the resins which 

 possess this peculiar quality except perhaps ice, and it is curious to 

 think that the same quality which in ice allows of that gradual creep- 

 ing and subsidence and consequent formation of glaciers with their 

 characteristic moraines, &c., will in pitch help us to produce 

 accui-ate optical surfaces. 



Polishing Machines. 



The two best-known polishing machines are those of the late 

 Earl of Rosse and the late Mr. Lassell, the general forms of which are 

 shown in these diagrams. Time will not permit me to enter into a 

 minute description, of their working, nor is it necessary, as both have 

 been often described. 



A few words, however, as to the different character of the strokes 

 given by these machines may be interesting. The stroke of Lord 

 Rosse's machine may be imitated in hand-work by the operator travers- 

 ing the polisher or mirror in a series of nearly straight strokes, of 

 about one-thii'd the diameter of the glass, to and from himself, at the 

 same time that he keeps walking slowly round the post, and instead of 

 allowing the centre of polisher to pass directly over the centre of 

 mirror, each stroke that he gives he slides a little (about one-tenth 

 diameter) to one side and then a little to the other. 



Mr. Lassell's stroke may be imitated by causing the polisher to 

 describe a series of nearly circular strokes a little out of the centre, 

 walking round the post at the same time ; thus the centre of polisher 

 will describe a series of epicycloidal or hypocycloidal curves on the 

 speculum. 



Many years ago my father devised a machine, figured and 

 described in Nichols' ' Physical Science,' by which either of these 

 motions could be obtained. He appeared to have got better results 

 with Mr. Lassell's strokes, for he afterwards devised a machine which 

 gave the same character of stroke, but over which the operator had 

 greater control, and this machine has been used for many years 

 with great success. Like all machines, however, which give a 

 series of strokes constantly recurring of the same amplitude, it is 

 apt to polish in rings. It is impossible to obtain absolute homo- 

 geneity in the pitch patches, and if any one square be a shade harder 

 than the general number, and that square ends its journey at each 

 stroke at the same distance from the centre of speculum or glass, 

 there will almost surely be a change of curvature in that zone. To 

 avoid this I have made a slight modification in the machine, which has 

 increased its efficiency to a great extent. I will now place in the 

 lantern a model of this machine, and first draw you a few curves with 

 the machine in its old state, and afterwards in its improved state. 



