522 ALVAN GRAHAM CLARK. 



called distinctively the Clark method. It reached its highest manifesta- 

 tion and refinement as used by the subject of this sketch, and may 

 briefly be described as follows. After a lens has been ground with 

 emery, it is polished with rouge on a bed of pitch or coal tar, the sphe- 

 rometer being frequently applied to see that it is approaching the 

 desired curvature. But, although the spherometer will easily detect an 

 irregularity of the fifty-thousandth of an inch, it is far too crude to satisfy 

 the highest demands of science, so that recourse must be had to some 

 other alternative, for even an irregularity of a few millionths of an inch 

 must not be disregarded. It is here that the method of local correction 

 comes in to remove the difficulty. The lens is now carried to the dark 

 room and tested on an artificial star, the eye being placed at the focus. 

 If the light is uniformly distributed over the whole surface and all the 

 rays come to a sharp point, the lens is in good figure, but such a condi- 

 tion would be extremely rare. Portions of the surface will almost inev- 

 itably be brighter than others, indicating slight irregularities of curvature. 

 These must be eliminated by returning the lens to the tool and remov- 

 ing the slight elevations by judicious polishing at these points. The lens 

 must also be frequently inserted in the telescope tube and tested on an 

 actual star at night. After this, the special local work begins. This 

 consists in taking some rouge upon the finger and changing the curvature 

 by delicate abrasion at those points which need correction, frequently 

 testing the lens to see that the desired result is being obtained. Here 

 the work is carried to millionths of an inch, for in a 6-inch lens of 90 

 inches focus, all the rays of light must be shot through a focal point ^^Vtt 

 of an inch in diameter. By tests with the polariscope, however, it may 

 be found that the glass is not quite homogeneous in density, though with- 

 out any definite flaw. In such a case the curvature must be changed to 

 obviate this condition, so that the best lenses are not always true curves. 

 As an instance of the amount of labor involved in local correction and 

 testing, it may be mentioned that the great Lick telescope of 36 inches 

 diameter was removed to the yard with its attendant risk, attached to 

 the great tube, and tried on stars on fifty-two different nights, beside all 

 the testing in the dark room. It becomes evident, then, that to see the 

 defects, judge correctly of their character, and remove them without incor- 

 porating others, demands the keenest eye, the most delicate judgment, 

 and exquisite manipulation, and it was here that Mr. Clark stood 

 unrivalled. 



Among the principal telescopes of the Clarks, and to which the sub- 

 ject of this sketch contributed the benefit of his judgment and skill, may 



