106 THE HISTORY OF THE TELESCOPE. 



the observed errors of color and lignre, until, by a purely tentative proc- 

 ess, the color error is practically negligible and the error of figure is 

 small. Then follows a process when the qualities of skill, conscientious- 

 ness, and perseverance have full scope. This process tirst introduced, or 

 at least made public by roucault, is known as local correcting. It 

 consists in slowly polishing away portions of the lens surfaces so that 

 errors in the focal image become so small, not that tliey can not be de- 

 tected, but that one can not determine whetlier they are on the one 

 side of truth or the other. Local correcting has always seemed to me 

 to be eminently unscientific and unnecessary. It is a process of mak- 

 ing errors small which ought not to exist. 



Mr. Brashear's method is essentially different from this. Before the 

 glasses are touched every dimension and constant of the finislied 

 objective is known with great accuracy. His whole aim is to make the 

 surfaces geometrically perfect; and by ingenious polishing machinery, 

 which embodies twelve years of his thought and experience, he is 

 enabled to do this with truly astonishing exactness. All the surfaces 

 which admit of investigaticm — usually three in his ordinary construc- 

 tion — are made rigidly true without regard to the character of the focal 

 image. This leaves only one surface which is known to l)e very nearly 

 a sphere, but probably deviating slightly within in tlie direction of a 

 l)rolate or oblate spheroid. A glance at the character of the focal image 

 will determine this point. Then the polishing machine is adapted to 

 bring about a change in the proper (lirecti<»n, and after action during a 

 measured interval of time, the image is again examined, and from the 

 observed change in character the necessary time for complete correc- 

 tion by the same or contrary action may be deduced. It will be 

 observed that by this means it is quite possible to correct errors which 

 are much too small to betray their nature, since a step in the wrong 

 direction carries with it no consequences of the slightest moment, since 

 any step may be retraced. 



When we learn that Mr. Brashear's telescope objectives have always 

 had a focal length differing only from one-tenth to one one-huudred- 

 and-eightieth of 1 per cent of the value prescribed, we have a sugges- 

 tion of the success of his efforts. But adding to that t\ie fact that he 

 is absolutely untrammelled by purely mechanical considerations, either 

 as to the shape of his lenses or the character of his materials, leaving 

 these questions to be decided alone by the requirements of the astrono- 

 mer, it seems to me that we may fairly accord to him the merit of the 

 most important improvements introduced into his art for a very long 

 period. 



I shall not venture to demand nnich of your time in considering the 

 purely theoretical difficulties in telescope construction, not merely be- 

 cause the subject has already taxed our patience, but because it would 

 be of almost too technical a character did we allow ourselves to regard 

 anything but the most general features. 



