July 26, :912] 



SCIENCE 



101 



glass to stand vertically, instead of hori- 

 zontally, and the disk is frequently tilted 

 up so as to submit the surface to optical 

 tests. These latter tests are far more 

 searching than those with a spherometer, 

 and enable the observer to detect an error 

 in the radius of curvature of portion of 

 the reflector of a hundredth of an inch. To 

 correct such an error it will be necessary 

 to remove a layer of glass of %oooooths of 

 an inch ! 



The most refined optical test is by the 

 observation of the image of a brilliant light 

 issuing :^rom a pin-hole close to the intended 

 center of the spherical surface. The ob- 

 server examines the image of the pin-hole 

 with a microscopic eye-piece placed as 

 close as possible to the pin-hole. He then 

 causes a straight-edge close in front of the 

 eye-piece to move slowly across the reflected 

 beam of light, either from left to right or 

 from right to left, so as to eclipse the light. 

 Previously to the eclipse the whole of the 

 glass seems to be a uniform blaze of light, 

 and if the curvature is perfect the light 

 which enters the observer's eye comes from 

 all parts of the disk, and the surface is 

 seen to darken equably all over. But if the 

 surface is imperfect the light from some 

 part is eclipsed sooner than that from 

 others, and the disk seems to possess con- 

 siderable hills and valleys illiiminated, as 

 it were, by a setting sun. 



The interpretation of these apparent 

 hills and valleys shows where further local 

 polishing with a small tool is requisite. Sir 

 Howard Grubb says that if he suspects a 

 hollow, he holds his hand near the surface 

 for a minute or two ; if a hill is suspected, 

 he washes the region with an evaporating 

 wash. The warmth in the one case and the 

 cooling in the other tend to rectify, and 

 indeed over-rectify, the errors. 



"When success is finally attained, after 

 all we have only a spherical surface, and it 



becomes necessary to obtain a parabolic 

 form. This last stage is done by further 

 tests of the kind described, with a dia- 

 phragm placed over the mirror which only 

 permits the observer to see the light re- 

 flected from chosen zones of the mirror. 

 The time at my disposal will not allow me 

 to describe this in further detail, or to tell 

 you how there is always found to be one 

 definite diameter of the glass along which 

 its weight must be supported. I must pass 

 by, too, the system of counterpoised levels 

 used for supporting the back of the glass, 

 and the method by which silver is chemi- 

 cally deposited on its surface. Meager al- 

 though this sketch has been, it will have 

 served to show you how beautiful are the 

 processes employed, and I would ask you 

 to realize that at first Herschel was a mere 

 amateur, and had to discover everything 

 for himself. 



As I have said, Hershel had to do all his 

 polishing by hand, and he found when once 

 the final stage had begun, it was necessary 

 that it should never stop even for a mo- 

 ment. Caroline relates how she was kept 

 busy in attending on her brother when 

 polishing : 



Since by way of keeping him alive I was con- 

 stantly obliged to feed him by putting the victuals 

 by bits into his mouth. This was once the case, 

 when in order to finish a 7-ft. mirror, he had not 

 taken his hand from it for sixteen hours together. 



The making of the mirror is, however, 

 but a small part of the difficulty of making 

 a telescope, for it involves high engineering 

 skill to provide a solid stand, an observing 

 platform, the graduated circles in right 

 ascension and declination for setting the 

 telescope and the clock, whereby it is made 

 to follow the stars in their daily motion. 

 The great size of Herschel's mirrors and 

 the weight of the long tube introduced 

 mechanical difficulties which were at that 

 time entirely new. 



