1886.] on Telescopic Objectives and Mirrors. 427 



so slight that it appears problematical whether an alteration would 

 really improve matters or not. Or, the observer may not be able to 

 make up his mind as to the exact position of the zone he suspects 

 to be too high or too low, and he fears to go to work and perhaps do 

 harm to an objective on which he has spent months of labour, and 

 which is almost perfect. In many such cases I have wished for some 

 means by which I could temporarily alter the surface and see it so 

 altered before actually proceeding to abrade and perhaps spoil it. 



During my trials with the great objective of Vienna, I thought of 

 a very simple expedient, which efifects this without any chance at all 

 of injuring the surface. If I suspect a certain zone of an objective 

 is too low, and that the surface might be improved by lowering 

 the rest of it, I simply pass my hand, which is always warmer than 

 the glass, some six or eight times round that particular zone. The 

 effect of this in raising the surface is immediately apparent, and is 

 generally too much at first, but the observer at eye end can then 

 quietly watch the image as the effect goes off, and very often most 

 useful information is thus obtained. The reverse operation, that of 

 lowering any required part of the surface, is equally simple. I take 

 a bottle of sulphuric ether and a camel's-hair brush, and pass the 

 brush two or three times round the part to be lowered, blowing on it 

 slightly at the same time ; the effect is immediately perceived, and can 

 always be overdone if required. 



So far then for the diagnosis. Now for the remedy. When the 

 fault has been localised, the lens is again put upon the machine and 

 the polisher applied as before, the stroke of the machine and the 

 size of the pitch patches being so arranged as to produce, or tend to 

 produce, a slightly greater action on those parts that have been found 

 to be too high (as before described while treating of the polishing 



The regulation of the stroke, excentricity, speed, and general action 

 of the machine, as well as the size and proportion of the pitch 

 squares, and the duration of the period during which the action is to 

 be continued, are all matters the correct determination of which 

 depends upon the skill and experience of the operator, and con- 

 cerning which it would be impossible to formulate any very definite 

 rules. All thanks are due to the late Lord Eosse and Mr. Lassell, 

 and also to Dr. de la Eue, for having published all particulars of the 

 process which they found capable of communication ; but it is a 

 notable fact that, as far as it is possible to ascertain, every one who 

 has succeeded in this line has done so, not by following written 

 or communicated instructions, but by striking out a new line for 

 himself; and I think I am correct in saying that there is hardly 

 to be found any case of a person attaining notable success in the art 

 of figuring optical surfaces by rigidly following directions or instruc- 

 tions given or bequeathed by others. 



There is one process of figuring which is said to be used with 

 success among Continental workers. I refer to the method called the 



