424 Mr. Howard Gruhh [April 2, 



In order to convey some idea of the relative quantities of material 

 removed by the various processes, I have placed upon the walls a 

 diagram which will illustrate this point in two distinct ways. 



The diagram itself represents a section of a lens of about 8 inches 

 aperture andl inch thick, magnified 100 times, and shows the relative 

 thickness of material abraded by the four processes. 



The quantity removed by the rough grinding process is repre- 

 sented on this diagram by a band 25 inches wide, the line grinding 

 by one -^^ inch wide, the polishing by a line -^q inch wide, while the 

 quantity removed by the figuring process cannot be shown even on 

 this scale as it would be represented by a line only x^oo i^^^ 

 thick. 



I have also marked on this diagram the approximate cost of 

 abrasion of a gramme of material by each of the four processes, 

 viz : — 



Figuring and Testing. 



By the figuring process, I mean the process of correcting local errors 

 in the surfaces, and the bringing of the surfaces to that form, what- 

 ever it may be, which will cause the rays falling on any part to be 

 refracted in the right direction. When an objective has undergone 

 all the processes I have described, and many more which are not so 

 important, and with which I have not had time to deal, and when the 

 objective is centered and placed in its cell, it is, to look at, as perfect 

 as it will ever be, but to look through and use as an objective it may 

 be useless. The fact is that when an objective has gone through all 

 the processes described, and is in appearance a finished instrument, I 

 look upon it as about one-fourth finished. Three-fourths of the work 

 has probably to be done yet. True, sometimes this is by no means 

 the case, and I have had instances of objectives w^hich were perfect on 

 the first trial ; but this is, I am sorry to say, the exception and not the 

 rule. 



This part of the process naturally divides itself into two distinct 

 heads : — 



1. The detection and localisation of faults — what may, in fact, be 

 termed the diagnosis of the objective. 



2. The altering of the figures of the difi'erent surfaces to cure 

 these faults. This may be called the remedial part. 



It may be well here to try to convey some idea of the quantities we 

 have to deal with, otherwise I may be misunderstood in talking of 

 great and small errors. 



